Chapter 2

The Large Hadron Collider and the

ALICE

In this chapter a brief overview of the Large Hadron Collider (LHC) is presented both

for p + p and heavy ion operational mode. The key design features of the various

subdetectors of ALICE are also described. As this thesis deals with an analysis of

data using subdetectors PMD and FMD, more details of these two subdetectors are

presented.

2.1 The Large Hadron Collider (LHC)

The LHC at CERN [1, 2, 3] is the largest particle accelerator in the world. The LHC

accelerator complex takes the advantage of the tunnel and some pre-existing particle

accelerator facilities that formerly housed the Large Electron Positron collider (LEP).

The LHC physics program is mainly aimed at proton-proton (p + p) collisions and

Pb + Pb collisions.

The LHC is a two ring superconducting collider. It accelerates two counter rotat-

ing beams in two separate beam pipes. It is 27 km in circumference and is equipped

with 1232 super conducting magnets which bend the beam trajectory. Eight radio

frequency (RF) cavities per beam deliver radio frequency power to accelerate the

beams, keep the bunches of particles well localized and compensate for energy loss

due to synchrotron radiation.

The layout of the LHC is shown in Fig. 2.1 and 2.2. It is segmented into eight oc-

tants. Each octant has a straight section in its center, the center of which is identified

numerically as a point (the number corresponding to the octant number).The arcs

are called Sector, denoted by xy where x and y are the numbers of adjacent octants

in clockwise order. For instance Sector 12 represents the arc between octants 1 and

2. Out of the eight point, the beams cross only at four points i.e points 1, 2, 5 and 8.

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Figure 2.1: Schematic view of the LHC.

The particles are injected into the rings before point 2 and 8. The RF system which

accelerates the particles is located at point 4. The beam dumping system is located

at point 6. At point 3 and 7 collimation systems are placed that ’clean’ the beam by

removing particles that have either a too large spatial spread from the bunch center

(particles in the so called beam halo) or are too fast or too slow, thus separated in

momentum space. The cleaning prevents particles from being lost in an uncontrolled

fashion within the accelerator.

The protons (or ions) are pre-accelerated before entering the LHC. These are

produced at LINAC 2 (LINAC 3) and boosted in the BOOSTER (LIER). They are

then sent to the Proton Synchrotron (PS) and the Super Proton Synchrotron (SPS)

for further acceleration. From the SPS the proton (ion) beam is split into bunches

going in either direction of the LHC main ring [2]. The bunches of protons (or ions)

are then further accelerated to the desire energies. The expected reachable energy

per nucleon in the center of mass for p + p collisions is 14 TeV whereas the same for

Pb + Pb collisions is 5.5 TeV. Table 2.1 summarizes the kinetic energy of the protons

and Pbq+ ions at different stages in the acceleration.

One important beam parameter is the luminosity L and is defined as

L =fnN2

A(2.1)

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Figure 2.2: The Cern accelerator complex. Adapted from [4]. The figure is not toscale.

Table 2.1: Kinematic energy at different stages of acceleration for proton and Pb208

beams [3]LINAC2 / BOOSTER / PS SPS LHCLINAC3 LIER

Proton Ekin 50MeV 1.4GeV 25GeV 450GeV 7TeVPb208 Ekin 4.2MeV/u 72.2MeV/u 5.9GeV/u 176.4GeV/u 2.76TeV/u

Charge +27 +54 +82 +82 +82

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Table 2.2: Some of the LHC beam parameters.System L Time between

√s(TeV ) Bunches per

(cm−2sec−1) collisions beamp + p 1034 25ns 7TeV 2808

Pb + Pb 1027 100ns 5.5TeV 592

where n is the number of bunches in both beams, N is number of particles per

bunch, A is the cross-sectional area of the overlapping region, f being the revolution

frequency. The frequency of interactions of a given process can be calculated from

the corresponding cross-section , σ, and the luminosity: dN/dt = L/σ.

The design parameters foreseen for nominal operation for p + p and Pb + Pb col-

lisions, are summarized in Table 2.2. Currently there are six experiments going on in

the LHC project; a brief discussion on which is given below.

ALICE (A Large Ion Collider Experiment) [5] is a dedicated heavy ion experiment

designed to study strongly interacting matter. It has supreme particle identification

capabilities with good acceptance for particles with low transverse momenta. It is

located at point-2. It explores the phase transition to the quark-gluon plasma, the

related phase diagram, and its properties. ALICE will also study collisions of protons,

as a baseline for heavy ion measurements .This thesis is based on the ALICE exper-

iment described in details in next section. ATLAS (A Toroidal LHC Apparatus) [6]

and CMS (Compact Muon Solenoid) [7] are general purpose proton-proton detectors

that are built to cover the widest possible range of physics at the LHC. They are situ-

ated at point-1 and point-5 respectively. These experiments aim at the search for the

Higgs boson and physics beyond the Standard Model, e.g. search for new heavy par-

ticles postulated by supersymmetric extensions (SUSY) of the Standard Model and

evidence of extra dimensions. LHCb (The Large Hadron Collider beauty experiment)

[8] Located at point-8, the LHCb experiment aims at studies related to CP symmetry

violation processes in heavy b-quark systems. LHCf (Large Hadron Collider forward

experiment) [9], located close to the ATLAS experiment detects forward particles

created during LHC collisions to provide further understanding of high energy cosmic

rays. TOTEM (TOTal Elastic and diffractive cross-section Measurement) detector

[10] measures the total cross-section, elastic scattering, and diffractive processes. The

detector is located close to the CMS experiment.

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2.2 The ALICE detector

ALICE [5] has been designed to be a general purpose particle detector system, capable

of identifying most of the particles produced in p + p and specially Pb + Pb collisions

[5]. It has been optimized for the very high multiplicity environment that is expected

in central heavy ion collisions. The design was initially developed for dNch/dη = 4000.

Its key features were later optimized for dNch/dη = 8000 .

The detector’s unique features are the tracking and particle identification over a

large range of momenta, from tens of MeV/c to over 100 GeV/c. It is expected to

provide data for variety of physics topics ranging from soft to jet physics and high pT

particle production.

Figure 2.3: Schematic view of the ALICE detector.

The ALICE experiment consists of a central barrel detector system covering a

pseudorapidity range given by |η| < 0.9 with full azimuthal coverage.In addition

there are several forward and backward detectors. The central subdetector system is

contained in a large solenoidal magnet of the former L3 collaboration which generates

a field of 0.5 T parallel to the beam axis. The subdetector configurations are optimized

for the detection of hadrons, electrons, and photons. The central system includes,

in order of increase in radii from the interaction vertex, an Inner Tracking System

(ITS)[11], a Time Projection Chamber (TPC) [12], a Transition Radiation Detector

(TRD) [13] and a Time Of Flight (TOF) [14] detector. These four detectors cover

the central region with |η| < 0.9. The aim of these detectors is tracking and particle

identification in the very high multiplicity environment. The other central detectors

which cover smaller regions of phase space than the previously mentioned central

region are a PHOton Spectrometer (PHOS) [15], an ElectroMagnetic Calorimeter

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(EMCAL) [16], a High Momentum Particle Identification Detector (HMPID) [17]

and an Alice COsmic Ray DEtector (ACORDE).

The forward rapidity systems include, a Photon Multiplicity Detector (PMD) [18],

an ensemble of Forward Multiplicity Detectors (FMD) [19], a Zero Degree Calorimeter

(ZDC) [20], a system of scintillation and quartz counters denoted as V0 [19] and T0

[19], and a Muon spectrometer [21, 22]. The schematic view of the ALICE detector

is shown in Fig. 2.3.

2.2.1 The ALICE coordinate system

Figure 2.4: The ALICE coordinate system.

The coordinate system of ALICE [23] is shown in Fig. 2.4. The nominal interaction

point is shown as the origin x = y = z = 0. The x-axis is shown perpendicular to the

mean local beam direction, lying in the local horizontal plane of the LHC and pointing

to the accelerator center. The y-axis is perpendicular to the x-axis and the mean local

beam direction, pointing upward. The z-axis is parallel to one of the the mean local

beam directions (as shown in the Fig. 2.4). An observer looking along the positive z

direction has the accelerator center on the left. The muon arm is at negative z. The

PMD lies at 367.5 cm from the interaction point in the +ve z direction. Any particle

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emitted along a vector r has a polar angle θ which increases from 0 (emission along z)

to a maximum value of π corresponding to emission along −z. The azimuthal angle

φ which increases clockwise from the x-axis (φ = 0) passing through the y-axis (φ =

π/2) finally coming back to the x-axis (φ = 2π). This is for an observer standing at

negative z and looking towards the point of origin.

In the next section, a brief description of the aforementioned detectors, excluding

PMD and FMD is given. Further details of PMD and FMD are presented in section

2.5.

2.3 Central barrel

2.3.1 The Inner Tracking System (ITS)

The ITS is a high resolution silicon detector system consisting of six layers of detec-

tors with inner and outer radii from 3.9 cm to 43 cm. This is the detector system

closest to the interaction point. As mentioned earlier, its pseudorapidity coverage

corresponds to |η| < 0.9 for collision located within an interaction region of 10.6 cm

along the beam direction. It consists of three subdetectors, starting from the centre

and going outwards : a Silicon Pixel Detector (SPD), a Silicon Drift Detector (SDD)

and a Silicon Strip Detector (SSD). Each of the subdetectors have two layers.

The SPD is a silicon pixel detector. The two layers of SPD are placed at 3.9 cm

and 7.6 cm from the interaction point with an acceptance of |η| < 2.0 and |η| < 1.4

respectively. No energy loss information is available for these two layers, so the SPD

does not contribute to the particle identification. These layers of ITS measure the

charged particle multiplicity. The other two subdetectors of the ITS, the SDD and the

SSD have the energy loss information. They provide further tracking points enabling

charged particle multiplicity measurements along with particle identification.

The basic functions of ITS are as given below.

• Determination of the primary vertex and the secondary vertex needed for the

reconstruction of charmed hadron and hyperon decays, with a spatial resolution bet-

ter than 100µm.

• Particle identification and tracking of low momentum particles.

• To improve the momentum resolution for the high momentum particles detected

also in the TPC.

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2.3.2 The Time Projection Chamber (TPC)

The TPC is the main tracking detector of the central barrel. It is optimized to pro-

vide, together with the other central barrel detectors (ITS, TRD and TOF), charged

particle momentum measurements with good two track separation, particle identifi-

cation via dE/dx, and vertex determination. It has an inner radius of about 44 cm

, an outer radius of about 250 cm and an overall length of 500 cm along the beam

direction. It consists of a cylindrical field cage, filled with 90×106cm3 of Ne/CO2/N2

(90/10/5), in which the primary electrons are transported over a distance of up to

250 cm on either side of the central electrode to the end plates. Multi wire propor-

tional chambers with cathode pad readout are mounted into 18 trapezoidal sectors

at each end plate. The TPC can detect tracks from a low pT of about 0.1 Gev/c to

a high pT of 100 GeV/c with good momentum resolution. For tracks with full radial

track length (with matches in ITS, TRD, and TOF detectors), the pseudorapidity

acceptance of the TPC is |η| < 0.9. For tracks with reduced track length (at reduced

momentum resolution), there is acceptance up to about |η| < 1.5.

The TPC is the main detector for the study of hadronic observables in both proton

and heavy ion collisions. Through the study of hadronic observables, the TPC will

provide information on the flavour composition of the collision fireball and on its

space time extent at freeze out. In conjunction with the TRD and ITS it will be used

to study vector meson resonances, charm and beauty through the measurement of

leptonic observables.

2.3.3 The Transition Radiation Detector (TRD)

The main task of the TRD is to identify electrons from other charged particles espe-

cially at higher momenta greater than 1 GeV/c. It also contributes to the tracking of

particles. The TRD works on the principle of transition radiation which is emitted

when a charged particle crosses over the boundary between two materials with dif-

ferent dielectric constants. The amount of radiation emitted depends on the Lorentz

factor γ of the particle. However the overall probability to create transition radiation

at one media boundary is low, so many layers of the media boundaries are used as

a result more than one detectable transition radiation is produced for particles with

γ > 1000 [24]. The pseudorapidity acceptance of the TRD is same as that of TPC i.e.

|η| < 0.9. The TRD is located just outside the TPC at radii from 290 cm to 370 cm.

It consists of six radiator layers, which produce transition radiation when traversed

by relativistic particles, and of a wire chamber filled with Xe in which the transition

radiation is detected.

Along with the TPC and the ITS, the TRD provides good enough electron identi-

fication to study the production of light and heavy vector mesons resonance and the

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continuum in the dielectron spectrum both in p + p and Pb + Pb collisions.

2.3.4 The Time-Of-Flight Detector (TOF)

The main task of the TOF is to identify particles in the intermediate momentum

range, below about 2.5 GeV/c for kaons and pions , up to 4GeV/c for protons with a

π/K and K/p separation better than 3σ by measuring the time between the collision

and the arrival of the particles in the TOF where the dE/dx technique is no longer

effective to separate K and π mesons. It is located just outside of TRD with the

same pseudorapidity acceptance as TRD i.e. |η| < 0.9. Its inner and outer radii are

370 cm and 390 cm respectively. The TOF detector design is based on the Multi gap

Resistive Plate Chamber (MRPC) technology . The key aspect of this technology is

that the electric field is high and uniform over the whole sensitive gaseous volume of

the detector. Any ionization produced by a traversing charged particle immediately

starts a gas avalanche process which generates a signal on the pick up electrodes. The

time jitter is caused only by fluctuation of the avalanche growth as there is no time

drift associated to the movement of the electrons into a region of high electric field.

The main goal of the TOF is to study the QCD thermodynamics via the mea-

surement of π, K and p transverse momentum distributions and particle ratios on an

event by event basis and signatures of QGP formation via open charm and φ meson

production.

2.3.5 The PHOton Spectrometer (PHOS)

PHOS is a high resolution electromagnetic calorimeter. It is based on leadtungstate

crystals(PWO) designed to measure the temperature of collisions by detecting pho-

tons emerging from them. When high energy photons strike lead tungstate, they

make it glow, or scintillate. They are readout using avalanche photo diodes. Lead

tungstate is extremely dense (denser than iron), stopping most photons that reach

it. PHOS is located at a radius of 460 cm and covers a pseudorapidity acceptance of

|η| < 0.12 with an azimuthal acceptance as given by 220 < φ < 320. This covers

about 3.7% of the central phase space region. A set of multi wire proportional cham-

bers in front of the PHOS is used to reject the charged particle. This is called the

Charged Particle Veto (CPV).

The main physics objectives of PHOS are the test of thermal and dynamical

properties of the initial phase of the collision extracted from the low pT direct photon

measurements and the study of jet quenching through the measurement of high pT

π0 and γjet correlation.

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2.3.6 The ElectroMagnetic Calorimeter (EMCal)

The EMCal is a large Pb scintillator sampling calorimeter which aims to measure the

transverse energy of (ET ) and transverse momentum (pT ) of particles that hit the

detector. The EMCal provides ET measurement in the region from 100 MeV to 100

GeV. It also provides a fast and efficient trigger (L0, L1) for hard jets, photons and

electrons. The EMCal is cylindrical in shape, located at a radius of 450 cm from the

beam line approximately opposite to the PHOS. It covers a pseudorapidity interval

of |η| < 0.7 over an azimuthal acceptance as given by 80 < φ < 187 which is about

23% of the phase space of the central region . Though it has an acceptance larger

than PHOS, it has lower granularity and resolution.

2.3.7 The High Momentum Particle Identification Detector

(HMPID)

The main aim of the HMPID is to enhance the particle identification capability of

ALICE by enabling identification of charged hadrons beyond the momentum interval

attainable through energy loss in ITS and TPC and time of flight measurements in

TOF.It allows the inclusive measurement of charged particle with momenta 1 - 5

GeV/c. It covers a pseudorapidity acceptance of |η| < 0.6 and azimuthal acceptance

of 1.2 < φ < 58.8 which is about 5% of the central region phase space. The HMPID

is based on proximity focusing Ring Imaging Cherenkov (RICH) counters. It consists

of a layer of radiator material of low chromaticity C6F14 liquid. Cherenkov photons,

emitted by a fast charged particle traversing the radiator are detected by a photon

counter.

2.3.8 The ALICE Cosmic Ray (ACORDE)

The ACORDE is an array of 60 identical plastic scintillator counters. It is placed

on the top of the ALICE magnet. It provides a fast (Level 0) trigger signal, for

the commissioning, calibration and alignment procedure of some ALICE tracking

detectors. The pseudorapidity acceptance of the ACORDE is |η| < 1.3 with azimuthal

coverage |φ| < 60.

Table 2.3 summarizes the acceptance, location and independent electronics read-

out channels of the ALICE central barrel detector system.

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Table 2.3: Summary of the ALICE central detector subsystems. The acceptance inη is calculated from the nominal interaction point. There is full azimuthal coverageunless noted otherwise. The position is given in terms of the distance from theinteraction point to the face of the detector. It corresponds to the radius for barreldetectors (inner and outer radius for the TPC and TRD) or the position along thebeam (z coordinate) for the other detectors. The dimension corresponds to the totalarea covered by active detector elements. Channels is the total number of independentelectronic readout channels. In case a detector is subdivided, the numbers refer tothe individual components (e.g. pixel layers 1 and 2)

Detector Acceptance (η, φ) Position (cm) Dimension (cm2) Channels

ITS layer 1, 2 ±2, ±1.4 3.9, 7.6 0.21 × 104 9.8 × 106

(SPD)ITS layer 3, 4 ±0.9, ±0.9 150, 239 1.31 × 104 133000

(SDD)ITS layer 5, 6 ±0.97, ±0.97 380, 430 5.0 × 104 2.6 × 106

(SSD)

TPC ±1.5 at r = 140 cm 84.8, 246.6 readout 32.5 × 104 557568±0.9 at r = 280 cm Vol. 90 × 106 cm3

TRD ±0.84 290, 368 716 × 104 1.2 × 106

TOF ±0.9 378 141 × 104 157248

HMPID ±0.6 500 10 × 104 1612801.2 < φ < 58.8

2.4 Forward Detectors

2.4.1 The V0 Detector

The V0 detector is a small angle detector consisting of two arrays of scintillator

counters called V0A and V0C, which are located at z = 340 cm (opposite to the

muon arm) and -90 cm from the interaction point. The V0A has a pseudorapidity

acceptance of 2.8 < η < 5.1 whereas the V0C covers a range −3.7 < η < −1.7. Both

V0A and V0C provide minimum bias triggers to the central barrel detectors both in

p + p and Pb + Pb collisions. The V0 detector also provides multiplicity information.

2.4.2 The T0 Detector

The T0 is a high resolution timing detector. It measures the collision time with a

precision of 25 ps. It measures the vertex position with a precision of ± 1.5 cm for

each interaction. If the vertex position occurs inside a window where interactions

are excepcted then an L0 trigger is issued. A signal corresponding to the vertex

53

position outside the above collision region is used as beam gas rejection signal. The

T0 can also generate an early weak up signal to the TRD, prior to L0. In addition,

T0 provides redundancy to the V0 counters and can generate minimum bias and

multiplicity triggers.

The T0 consists of two arrays of Cherenkov counters, called T0A and T0C. The

T0A is located at z = 375 cm from the nominal vertex, opposite to Muon detector.

The pseudorapidity range of T0A is 4.61 < η < 5.92. The other component of T0

viz the T0C is located at z = −72.7 cm from the nominal vertex with pseudorapidity

range of −3.28 < η < −2.97.

2.4.3 The Zero Degree Calorimeter (ZDC)

Two identical sets ZDCs are located at 116 m on either side of the Interaction point.

They provide an estimation of the impact parameter of heavy ion collisions by the de-

tection of the spectator nucleons. The measurement is preformed by the two calorime-

ter, one for neutrons called ZN with |η| < 8.8 and another for protons called ZP with

6.5 < η < 7.5. At this distance from the interaction point protons are well separated

from neutrons by the magnets in the beam line. The measurement is complemented

by an electromagnetic calorimeter called ZEM (4.8 < η < 5.7) kept at z = 7.5 m.

This measures the total forward energy.

2.4.4 The Muon Spectrometer

The task of the Muon spectrometer is to detect dileptons, and extract all possible

physics from those measurements, including J/Ψ suppression, ρ mass broadening

etc. The production of open charm and beauty can also be studied using the muon

spectrometer. The spectrometer is located on the -ve z (opposite to the PMD)of

the ALICE experiment. It accepts particles in -4 < η < -2.5 and has full azimuthal

coverage for muons with p > 4 GeV/c. This cut off is due to the fact that to reach

the spectrometer muons first have to pass through the front absorber made of carbon,

concrete, and steel. Successively they are measured by five tracking stations with two

planes each made of very thin, high granularity, cathode strip tracking stations. A

dipole magnet with an integrated magnetic field of 3 Tm is located outside of the

L3 magnet to allow the muons momenta to be reconstructed. Two tracking stations

are located in front of the dipole magnet. One tracking station is in its center; two

are positioned behind the magnet. An iron wall of 1.2 m acts as a further muon

filter after which two trigger stations with two planes each of resistive plate chambers

are located. The whole spectrometer is shielded by means of a dense absorber tube

against particles emerging from the beam pipe.

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2.4.5 The Photon Multiplicity Detector (PMD)

The PMD has been installed to measure the photon multiplicity in the forward ra-

pidity region of the ALICE experiment. It is at a distance of 367.5 cm from the in-

teraction point opposite to Muon spectrometer. PMD covers a pseudorapidity range

as given by 2.3 ≤ η ≤ 3.9 with full azimuthal coverage.The detector consists of a

preshower and a Charge Particle Veto(CPV) plane separated by a 3X0 thick con-

verter system (1.5cm Pb+0.5 cm Stainless steel). The full PMD has 24 Unit Modules

(UMs) on each plane. The full detector has been fabricated in the form of two halves

which when joined form a close rectangular plane all around the beam pipe. The two

halves can be opened (separated) to gain access during maintenance/installation of

new systems. Each half of the PMD has 24 UMs (12 in CPV and 12 in the preshower

plane). As has been said earlier, PMD measures the multiplicity and the spatial dis-

tribution of photons on an event by event basis. However the CPV plane of PMD

along with one ring of the FMD i.e FMD2i can be used to measure the charged

particle multiplicity in a common phase space of both CPV and FMD2i.

2.4.6 The Forward Multiplicity Detector (FMD)

The FMD measures the charged particle multiplicity over a large fraction of phase

space, −3.4 < η < −1.7 and 1.7 < η < 5.0 with full azimuthal coverage. It consists

of 3 subdetectors, FMD1, FMD2 and FMD3. FMD1 consists of one ring (inner).

Therefore it is called FMD1i. On the other hand FMD2 and FMD3 consist of two

rings each. They are called FMD2i, FMD2o, FMD3i and FMD3o respectively where

i and o stand for inner and outer rings. Regarding position, FMD1 and FMD2 are

placed opposite to the Muon spectrometer (on the +ve z side). On the other hand

FMD3 sits on the -ve z side, same as the Muon spectrometer. The inner and outer

rings are segmented into 20 and 40 sectors in φ respectively. Each of the sectors of

inner and outer rings are segmented into 512 and 256 strips of Si as detectors. So

each ring consist of 10,240 (total number of strips in a ring) independent readout

channels. Such a segmentation of the FMD was chosen such that, on an average, one

charged particle would occupy each strip for central events.

2.5 More details on PMD and FMD

As mentioned earlier the present thesis deals with data on Nγ and Nch (photon and

charged particle multiplicities) measurements at ALICE for p+p collisions at 900 GeV.

These data were taken by the subdetectors PMD and the FMD placed in the forward

rapidity region. Therefore it is important to make somewhat detailed presentations

on the above two forward detectors. This is presented in the next two subsections.

55

Since we were also concerned with the fabrication installation and calibration of the

PMD, these aspects are also presented but in later sections.

2.5.1 The Photon Multiplicity Detector (PMD)

The PMD has been used to measure the multiplicity and the spatial distribution of

photons in the ALICE experiment. This detector consists of two identical planes

of hexagonal cell gas detectors separated by a Pb converter. The front plane is a

charge particle veto (CPV) detector the one at the back corresponding to a preshower

detector for detecting photons. The PMD sits in the forward rapidity region at z =

367.5 cm, with 2.3 < η < 3.8 and full azimuthal coverage.

Physics goals

From the measurement of Nγ and the spatial distribution of photons, on an event by

event basis, together with the informations obtained from other detectors, PMD is

expected to investigate the following broad topics of physics :

(A) Determination of the reaction plane and probe of thermalisation through

studies of azimuthal anisotropy and flow.

(B) Critical phenomenon near the phase boundary leading to fluctuations in global

observables like multiplicity (Nγ) and pseudorapidity density of photons.

(C) Study of Chiral symmetry restoration (e.g., disoriented chiral condensates)

through the measurement of charged particle multiplicity (Nch) from FMD in a com-

mon part of phase space and study of the observables Nγ and Nγ/Nch with full

azimuthal coverage.

Principle of photon detection

The basic principle of the measurement of photon multiplicity using the PMD is

similar to those of preshower detectors used in WA93 and WA98 experiments at

CERN SPS [25] and STAR experiment at BNL [26] . In a preshower detector, a

photon on passing through the converter produces an electromagnetic shower through

the processes of pair production and bremsstrahlung radiation. These shower particles

produce signals in several cells of the sensitive volume of the detector which is highly

segmented. Charged hadrons usually affect only one cell and deposit very little energy

to produce signal. This is because of their low interaction cross-section with the

converter material. The signal produced by the charged particles are similar to those

produced by a minimum ionizing particles (MIP). A schematic diagram showing the

basic principle is shown in Fig. 2.5. The thickness of the converter is optimized such

that the conversion probability of photons is high and transverse shower spread is

small to minimize shower overlap in a high multiplicity environment.

56

CLUSTERHADRON

LEAD CONVERTER

VETODETECTOR

PRESHOWER DETECTOR

HONEYCOMB ARRAY

PHOTONCLUSTER

CONVERTERSUPPORT

HADRON

PHOTON

Figure 2.5: Basic principle of a preshower detector.

In addition to the preshower plane there is a CPV plane with an identical di-

mension and granularity as those of the preshower detector, placed in front of the

converter which acts as a veto for charged particles. The two planes are kept back to

back.

Each plane of the PMD is based on a proportional counter design. For the active

volume it was decided to use a mixture of Ar and CO2 with a ratio of 70:30 by

weight as fixed from previous experiments. This gas mixture is preferred because

of its insensitivity to neutrons. To handle the high particle density in the forward

region, the detector technology has been chosen with the following considerations.

(i) Multihit probability should be less.

(ii) Charged hadron signal should be contained in one cell.

(iii) Low energy δ-electrons should be prevented from traveling to nearby cells and

causing cross talk among adjacent cells.

Requirements of high granularity and isolation of cells demand a segmentation

of the detector volume, with material effective for reducing δ-electrons from crossing

one cell to the other. To have the best packing a honeycomb cellular geometry was

selected with wire readout.

Making of the PMD

The PMD has two planes. Each plane has 24 Unit Modules (UM). Each of the UM

consists of an array of hexagonal/honeycomb cells. There is a total 4608 (48x96)

number of cells in a UM. The diameter of the inscribed circle and the depth of each

honeycomb cell are 5.0 mm each. They are made out of Cu strips 0.02 mm thick. A

57

hexagonal cell is shown schematically in Fig. 2.6. The cells were made with notches at

corners for a smooth flow of gas between them. An UM has the following components

0.3mm

Cathode

2 mmAnode

Extended portion of cathode

Cell depth = 5 mm Insulation circle

wire support

Figure 2.6: Schematic diagram of the cross section of a unit cell of the PMD.

:

• An array of 48 × 96 or 96 × 48 cells

• Two Printed Circuit Boards (PCBs). One is in the front the other one being at

the back.

• A gas tight encloser stainless steel (SS) frame.

• A high voltage box.

Honeycomb ChambersA honeycomb array of cells fabricated in the factory has 576 hexagonal cells (12

columns × 48 rows). Eight of these honeycomb arrays were joined together for making

a full array of cells as required for making a UM. There are two kinds UMs, one long

type and one short type, made using two different combinations of the above eight

smaller arrays. The short type array has 48 rows and 96 columns arranged inside a

rectangle of length 419 mm and width 242.5 mm. The long one has 96 rows and 48

columns arranged in a rectangle of dimension 482.5 mm x 210 mm.

Mounting of Front and Back PCBs on a honeycomb chamberTwo Au plated printed circuit boards (PCB) with metalized inner surfaces, each

having 4608 insulation circles of 2 mm diameter formed the front and back surface of

the honeycomb chambers. The top PCB has solder islands at the center correspond-

ing to each cell with a 0.3 mm diameter gold plated through hole. Signal tracks from

58

a group of 32 cells are brought to a 32-pin connector. There are 144 such connectors

in each honeycomb chamber corresponding to a UM. The PCBs on the bottom side

have only soldering islands without signal tracks, serving as anchor points. The inner

part of the PCBs are gold plated, with circular islands near the anode wire. Together

with the honeycomb wall they form part of an extended cathode going very close to

the anode wire. The PCBs are different for different type of honeycomb chamber

(short or long type). Fig. 2.7 shows the different component of a UM. A honeycomb

Figure 2.7: Schematic view of components of unit module. 1) Fixing screw; 2) 32 pinFRC connector; 3) Top PCB; 4) Honeycomb chamber; 5) Bottom PCB.

chamber is sandwiched between two PCBs by making proper alignment. For a given

UM, one of the alignment pin is used to provide high voltage (HV) connection to the

honeycomb walls. The honeycomb body along with the Au deposited front and back

surface formed the cathode. Au plated tungsten wires of diameter 20 µ, soldered

between the the two PCBs passing through the center of the honeycomb cells, served

as the anodes.

Assembly of UMThe UM were prepared keeping the honeycomb chambers inside a gas tight en-

closure. A 2 mm thick FR4 grade glass epoxy sheet formed the base plate of a UM,

59

fixed to a 2 mm thick SS frame. A plate for high voltage box and two nozzles for gas

inflow and outflow were fixed on the SS frame for both long and short type modules.

Support plateAs we had mentioned earlier, PMD has two parts on both sides of the beam line.

A 5 mm thick SS is used to support the lead converter plates and the UMs in each

half of the PMD. The SS plate has tapped holes for screws corresponding to hole

position in the lead converter plates. There are two different slots on the SS plate for

placing two different type of modules . The complete support plate has two asymmet-

ric pieces which when joined together form an approximate rectangle. The SS plate

on each side has 12 UMs in each plane (24 in total). The support mechanism for

PMD is shown in Fig. 2.8 The edges of the support plate are also used for mounting

Figure 2.8: PMD support mechanism. The inner rectangular part shows the twohalves. The two halves, when separated for servicing, look as shown on the right andleft.

the gas feed manifolds, shoe boxes for low voltage supplies and general support for

distribution of cables onto the detector.

Converter plate

60

Two different types of rectangle shaped Pb converter plates for two different types

of modules were fixed on the SS support plate with the help of screws. The PMD

is assembled in two equal halves. Each half has independent cooling, gas supply and

electronics accessories. The PMD is supported from a stainless steel girder which

forms part of a baby space frame in the forward region. Provision has been made for

both x and z movements. The two halves can be moved on the girder to bring them

together for data taking operation or separated for servicing. The girder itself can be

moved on the baby space frame to insert the PMD in the solenoid magnet.

Front-End Electronics (FEE)and Read out for PMD

A schematic diagram of the FEE of the PMD is shown in Fig. 2.9, which is similar to

the setup for the tracking chambers of ALICE muon spectrometer [22]. A photograph

of the FEE board is shown in Fig. 2.10. The signals from the anode wires for a group

of 64 cells within a matrix of 4 rows and 16 columns are connected to two 32-pin

connectors by a flexible cable which connects to a FEE board at the other end.

The signals are processed using a multiplexed analog signal processor (MANAS) chip

which handles 16 channels providing multiplexed analogue outputs. Each FEE board

has four MANAS chips, two 12-bit ADCs, and a custom built ASIC called MARC chip

(Muon Arm Readout Chip, developed for the Muon spectrometer) which reads out

all 64 channels. The MARC chip controls 4 MANAS chips, two ADCs and performs

zero suppression of the data.

A set of FEE boards are read out using Digital Signal Processors (DSP). The

MARC chip communicates with DSP through a 4-bit bus. The DSPs are handled

through a cluster readout system called the CROCUS (Cluster ReadOut Concentrator

Unit System). The main objectives of the CROCUS are to gather and concentrate the

information coded on the FEE and pass on to the data acquisition (DAQ) system,

drive the FEE boards via patch bus controllers, receive and distribute the trigger

signals, and perform calibration of the detector. Each CROCUS crate can handle 50

patch-buses. Each patch-bus handles one chain of FEE boards. The readout chain

is designed considering the occupancies in the chains and the restrictions on chain

length as dictated by faithful signal transmission. The chain arrangement for CPV

and preshower planes are same. Each readout chain in one PMD plane (preshower or

CPV) has 12 FEE boards. Each UM has 6 chains. Therefore each half of one PMD

plane (preshower or CPV plane) has a total of 72 chains. However only 50 of them

corresponding cells with η > 2.3 are connected to one CROCUS. Another CROCUS

controls the other half of the same plane. Similarly two other CROCUS modules

control the two halves of the other PMD plane.

61

Figure 2.9: Front-End Electronics architecture of the PMD.

Some Physical Parameters of the PMD

For making the PMD, several important parameters like granularity of the detector,

acceptance of the detector, converter thickness, operating voltage range, operating gas

mixture, response to hadrons and photons all need to be understood and optimized.

Out of these parameters position and granularity of the detector are decided by the

physics requirements and high particle multiplicity at LHC. On the other hand most

of the other parameters are fixed based on the experimental test beam data with

electron and hadron beams obtained on small PMD prototypes at CERN PS. All

these parameters have been optimized for handling the large particle multiplicity at

LHC.

Converter Thickness and Cell SizeThe converter plays an important role in the preshower detector. With increase

in the thickness of the converter, there will be an increase in the preshower signal for

an electromagnetic particle. Such an increase in the converter thickness results in a

transverse spread of the shower size. In a high multiplicity environment there is a

62

Figure 2.10: Photograph showing both sides of the Front-End Electronics board ofthe PMD. Four MANAS chips along with the MARC are clearly seen.

η1.5 2 2.5 3 3.5 4 4.5

Azi

mut

hal a

ccep

tanc

e (%

)

0

20

40

60

80

100

Figure 2.11: Azimuthal acceptance (φ) of PMD as function of pseudorapidity η.

increase in multihit probability in a single cell. In view of this and based on earlier re-

sults [25, 26], it was decided to use a 3X0 thick Pb converter for the preshower plane.

This thickness maps to about 1.5 cm of Pb together with a 0.5 cm of thick SS layer.

The granularity of the ALICE-PMD was optimized by taking the maximum particle

multiplicity in an event expected at LHC (dN/dη = 8000 at η = 0). A uniform cell

size of 0.22 cm2 has been selected for both veto and preshower plane of the PMD [27].

AcceptanceThe azimuthal (φ) coverage of ALICE PMD as a function of pseudorapidity (η)

is shown in Fig. 2.11. This has been obtained by randomly generating particles with

an η range of 1.5 to 4.5. It must be mentioned that there is a nearly square opening

63

(GeV)incE-110 1

Con

vers

ion

Effi

cien

cy (

%)

0

10

20

30

40

50

60

70

80

90

100

Figure 2.12: Photon conversion efficiency as a function of photon incident energy.

of dimension 10.5 cm x 12 cm at the center of the PMD for the beam pipe to go

through. The lower dimension of 10.5 cm of the opening decides the maximum η

for the PMD which is 4.5 units. Similarly the minimum η for PMD corresponds to

one of the outside corners and is 1.5 units. The φ coverage was estimated taking the

percentage of particles falling within the PMD azimuthal acceptance in a particular

η window. It is clearly seen from the Fig. 2.11 that the PMD has full azimuthal (φ)

coverage within 2.3 ≤ η ≤ 3.8.

Photon Conversion efficiencyPMD has a 3 radiation length (X0) of material (Lead + steel) in front of the

preshower plane. Photon being an electromagnetic particle is converted into electro-

magnetic shower (electron and positron). The shower particles get detected in the

preshower plane of the PMD. Low energy photons may get absorbed in the converter

and hence will not be detected in the preshower plane of the PMD. Photon conver-

sion efficiency is defined as the ratio of number of photons incident on the converter

material (3X0 of lead+steel in our case) to the number of photons which give signal

in the preshower plane of the PMD above a certain noise threshold .

The conversion efficiency (shown in Fig. 2.12) is calculated by using single incident

photons of various energies in simulation. This (conversion efficiency) puts an upper

limit on photon counting efficiency which depends on certain other factors that are

64

η2.4 2.6 2.8 3 3.2 3.4 3.6 3.8

Occ

upan

cy (

%)

0

0.2

0.4

0.6

0.8

1

Figure 2.13: Occupancy of ALICE PMD as a function of η from simulation for p + pcollisions at

√s = 900 GeV.

used to identify a photon cluster. This implies that, we can not have the photon

counting efficiency above the photon conversion efficiency. Fig. 2.12 shows the con-

version efficiency for photons as a function of incident energy. One sees that above

an incident energy of 1 GeV the photon conversion efficiency is about 90 % or higher

and independent of incident energy.

OccupancyThe occupancy for the PMD is defined as the ratio of number of cells hit to the

total number of cells. The occupancy is reflective of the granularity of the detector

and for a fixed granularity the particle density falling on the detector. The detector

configuration is designed to keep the occupancy to a lower level, so that efficiency

of particle identification and counting is good. Fig. 2.13 shows the occupancy of

ALICE-PMD as a function of η for p + p collisions from PYTHIA at√

s = 900

GeV. The increase in occupancy as we go from 2.3 to 3.7 in η, reflects the increase

in particle multiplicity per unit area. It means the ratio of the number of particles

falling on the detector to the number of cells in the η ∼ 3.7 region is higher compared

to corresponding number at η ∼ 2.3. The maximum occupancy in simulation for p+p

at 900 GeV is less than 1 %.

Table 2.4 summarises the physical parameters of the PMD in terms of acceptance,

location, independent electronics readout channels and operating parameters. The

65

Table 2.4: Summary of design and operating parameters of the PMD.Pseudorapidity coverage 2.3 < η < 3.8Azimuthal coverage 3600

Distance from vertex 3.67 mDetector active area 2.59m2

Detector weight 1200 kg

Number of planes 2 (Veto and preshower)Converter 3X0 (1.5 cm Pb + 0.5 cm SS)Hexagonal cell cross section 0.22 cm2

Hexagonal cell cell depth (gas thickness) 0.5 cmDetector gas Ar/CO2 (70%/30%)Operating voltage -1350 to -1400 VCharged particle detection efficiency 96%

Number of cells in one module 4608Number of modules per plane 24Total number of cells 221184Total number of HV channels 48

Number of FEE boards 3456Number of CROCUS crates 6Number of DDL channels 6

calibration and photon counting efficiency are given later in Chapters 3 and 4.

2.5.2 The Forward Multiplicity Detector (FMD)

The main function of the Forward Multiplicity Detector (FMD) is to provide charged

particle multiplicity information in the pseudorapidity range −3.4 < η < −1.7 and

1.7 < η < 5.0. At the same time the overlap between the FMD Si rings and the

ITS inner pixel layer provides redundancy and cross-checks of measurements between

subdetectors. It also ensures continuous coverage for a distribution of vertices along

the z-axis. As has been mentioned earlier, the FMD has five rings (three inner and

two outer) each with 10240 strips or readout channels. This azimuthal segmentation

of the FMD allows for the determination of the reaction plane for each event and the

analysis of flow within the FMD’s pseudorapidity coverage.

Simulations of central Pb + Pb collisions with dNch/dη = 8000 in the midrapidity

region were used to study the FMD design parameters [19]. Contributions from

secondary interactions in the beam pipe, ITS, T0 and V0 detectors and service plus

support structures, increase the number of particles impinging on the FMD. For these

events, the average hit density over the whole FMD is found to be about one charged

particle per strip. In addition no individual strip is found to have an average hit

greater than three charged particles. Peripheral A + A collisions and p + p collisions

produce significantly lower hit densities.

66

Table 2.5: Summary of η, φ and z position of the FMD rings.Sub- Ring φ- segments r - segments z rin rout

Detector (Sectors) (Strips) cm cm cmFMD1 I 20 512 320.0 4.2 17.2FMD2 I 20 512 83.4 4.2 17.2

O 40 256 75.4 15.4 28.4FMD3 O 40 256 -75.2 15.4 28.4

I 20 512 -68.8 4.2 17.2

On the hardware front the FMD is designed to allow up to 20 minimum ionizing

particles (MIPs) in a single strip before saturating. FMD has a readout time of

1.2ms which does not allow the FMD to serve as a multiplicity trigger and, therefore,

provides only offline analysis information.

Detector layout

Figure 2.14: Layout of the FMD rings in the ALICE experiment.FMD3 and FMD2are located on each side of the ITS detector while FMD1 is much further away fromthe interaction point (IP).

Fig. 2.14 shows the location of each FMD ring in ALICE as well as the basic

layout of the Si sensors within an FMD ring. FMD2 and FMD3, each consisting

of an inner and an outer ring of Si sensors, are located on either side of the ITS

detector at distances as given in table 2.5. FMD2 and FMD3 are positioned to

have approximately the same acceptance, however, the presence of the T0 detector

necessitated a different placement for the FMD3 inner ring. Another ring, FMD1,

67

was placed further from the interaction point opposite to the muon spectrometer to

extend the charged particle multiplicity coverage. The upper limit on this additional

coverage (determined to be at η = 5.0) is constrained by the increasing number of

secondary particle contributions at very forward rapidity.

Each detector ring consists of 10 (for an inner ring) or 20 (for an outer ring) Si

sensors, each sensor consisting of two azimuthal sectors. This is similar to a UM of

PMD. Each sensor is given a single HV which goes to all the strips in that sensor.

The inner radius of each of the rings is constrained by the beam pipe while the outer

radius is constrained by the inner radius of the TPC. The radial span (distance from

inner radius to outer radius) is limited to 15 cm corresponding to the diameter of

Si wafers from which the sensors are made. The radii of the strips in the inner ring

range from 4.2 cm to 17.2 cm corresponding to its inner and outer radii. Similarly the

radii of the strips in the outer ring lie between 15.4 cm and 28.4 cm, corresponding to

the inner and the outer radii of the outer ring. As has been mentioned earlier, each

of the rings (inner or outer) contains 10 240 Si strips giving the full FMD a total of

51200 Si strips (corresponding to 5 rings) to be read out. An overview of the FMD

design parameters are given in table 2.5 and 2.6.

Front-End Electronics and Readout

The signals from each Si strip in the FMD must be collected and transferred for

processing. The FMD module detects the particles and performs an initial analysis

of this information. A hybrid PC card with dimensions slightly smaller than the Si

sensor is attached to each sensor by gluing thin ceramic spacers between them to

form a single module. Bonding pads exist along the perimeter of both the Si sensor

and the hybrid PC card to allow small bond wires to collect the signals from the Si

sensor and deliver it to the readout electronics. To achieve the best signal resolution,

amplification of the signal must be done as close as possible to the detector element.

A VA preamplifier chip is placed directly on the hybrid PC card to amplify and

shape the detected signal. This preamplifier chip has low noise (250-350 ENC for

the FMD detector load) and a gain suitable for allowing signals up to 20 MIPs to be

read out before saturating. Each VA chip has input lines for individual amplification

and shaping of 128 signals. An inner Si module therefore requires eight VA chips

while an outer Si module requires four. A 40-pin connector on the hybrid PC card

provides enough connections for distribution of power to the VA chips, control of

readout, and dedicated lines for each VA chip’s differential readout of the amplified

signals. A separate high voltage connector is necessary for supplying the back plane

voltage to reverse bias the Si sensor and deplete the Si bulk. The hybrid PC card

also provides filtration circuitry to reduce noise on the high voltage line. Typical high

voltage values for the inner sensors are around 75 V while the outer sensors require

68

Table 2.6: Summary of design and operating parameters of the FMD.Pseudorapidity coverage -3.4 < η < -1.7, 1.7 < η < 5.0Azimuthal coverage 3600

Dynamic range 0-20 MIPSTotal channels 51200Dead channels < 0.1%Number of RCUs 3 (1 per ring system)

Inner ringNumber of channels 10240Number of digitizer cards 2Number of sensors 10Sensor inner radius 4.2 cmSensor outer radius 17.2 cmSector azimuthal coverage 180

Azimuthal sector per sensor 2Strips per azimuthal sector 512Strip pitch 250 µmBack plane voltage 75VVA chips per hybrid card 8

Outer ringNumber of channels 10240Number of digitizer cards 2Number of sensors 20Sensor inner radius 15.4 cmSensor outer radius 28.4 cmSector azimuthal coverage 90

Azimuthal sector per sensor 2Strips per azimuthal sector 256Strip pitch 500 µmBack plane voltage 130VVA chips per hybrid card 4

Si sensor thickness 3.25 ± 3 /mumSi bulk type n-typeBreakdown voltage > 250 VLeakage current per strip < 0.5 /muAStrip coupling capacitance 5-25 pFPolysilicon bais resistance 20 MΩ

69

around 130 V.

ALTRO(16 ch)

ALTRO(16 ch)

ALTRO(16 ch)

BoardController

and

cont

rol

Loca

l mon

itor

and

cont

rol

Loca

l mon

itor

Data proc.and memory

RCU(nrea patch pannel1 per sub−detector)

FMD Module(5 for inner , 10 forouter half−ring)

VA1 3

x 16

Trigger LVL0

(2.5 MHz < 0.05m)

Analog serial line

(40 Mhz ~ 3m)Front end bus2 for FMD1, 4 for FMD2 and 3)

(on detector, 1 per half ringDigitizer

Control network

VA1_3read−outcontrol

Sensor

Read−out

controller(ethernet)DCS if

(DDL−SIU)DAQ if

(TTC−rx)Trigger if TTC optical link

(clock, LVL1, LVL2)

DCS

DDL

room

In counting

Figure 2.15: Diagram of readout chain for the FMD. The board controller on the digi-tizer card controls the readout sequence of the Si modules. Analogue data is receivedat the ALTRO chips and digitized. The readout control unit (RCU) coordinates thetransfer of the digitized data from the ALTRO chips to the DAQ system.

Further electronics is needed to digitize the signals and control readout. An elec-

tronics digitizer board was built for this purpose. The digitizer board has the same

shape as a Si half ring and is mounted on the backside of the low mass honeycomb

shaped support plate of the Si modules. Short readout cables connect 5 inner modules

to an inner digitizer card or 10 outer modules to an outer digitizer card. The digitizer

card provides the low voltage to power the Si modules as well as controlling readout.

The main components of a digitizer card are the ALTRO chip [28] (three per digitizer

card), each used for digitizing the signal from one or two Si modules, and an FPGA

chip, which controls the readout of the Si modules as well as monitoring services for

temperature, voltage and current. The ALTRO chip is a fast ADC chip developed by

the ALICE TPC group. It allows analogue to digital conversion the detector signal,

thereby avoiding long cables for analogue signals. The use of the ALTRO chip in the

FMD allows further electronics in the readout chain to be common with the TPC.

The FPGA was used as a low cost means for controlling readout and monitoring.

Additionally, the use of an FPGA allows changes to be made to the readout and

monitoring algorithm without the requirement of physical to the hardware. For com-

plicated readout procedures such as calibration the FPGA can be programmed. A

schematic diagram of the readout chain is shown in Fig. 2.15. Transfer of data from

the ALTRO buffers to the DAQ system is controlled by the RCU.

Detector response

Fig. 2.16 shows the calibrated response for a single FMD strip to charged particles

accumulated over many events. The large peak at zero is generated from events where

70

(a)

(b)

Figure 2.16: The distribution of signals from a single Si strip is shown on the toppanel (a). The blue distribution corresponds to all signals seen in that strip. A largeset of signals having energies between the pedestal peak and the MIP peak is seen inthis distribution. The correlation between the signal seen in this strip and an adjacentstrip is shown on the bottom panel (b). A clear band is seen with signals whose sumof energy between the two strips is 1 MIP. These shared hits can be removed byrequiring the adjacent channels to have only noise (by making a maximum energycut) to reveal the actual detector response of Landau distributed signals (shown bythe fit).

71

no particle has deposited energy in this detector strip and the width of that peak arises

from the intrinsic noise of this detector strip along with the noise accumulated during

readout. While the energy per unit length deposited in a Si detector is described

by a Landau distribution, the total energy deposited in a detector strip also depends

on the angle of the particle relative to the detector. Additionally, some particles

share their deposited energy among multiple detector strips. After these shared hits

are removed from the distribution in Fig. 2.16, a peak is clearly seen separated from

the noise with a long tail with increasing energy. A fit of this peak to a Landau

distribution yields the most probable energy loss, that corresponding to a minimum

ionizing particle (1 MIP). Deviation of the fit from a Landau distribution is due to

incomplete removal of all shared hits. The ratio of this value to the noise (the signal

to noise ratio) determines the ease of distinguishing between hit and not hit strips.

Measurements performed with the FMD [29] have shown outer sensors to have an

average signal to noise ratio of 23:1 while inner sensors have an average ratio of 40:1,

far in excess of the design criteria of 10:1.

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