FMD

THE FORWARD MULTIPLICITY DETECTOR (FMD)

The Forward Multiplicity Detector (FMD) has a special role in ALICE. It is designed to measure the charged particles that are produced in the collisions and which are emitted at small angles relative to the beam line direction. The FMD is the primary detector to measure the multiplicity distribution of charged particles over a wide kinematical range  (rapidity coverage in the interval  1.7 < |eta| < 5.1 corresponding to an angular interval  of about 0.75 degrees to 21 degrees with respect to the beam direction.

The FMD team after successful installation of FMD in ALICE.

The FMD team after successful installation of FMD in ALICE.

THE PHYSICS TO BE STUDIED WITH THE FORWARD MULTIPLICITY DETECTOR

The FMD will, in addition to determining the total particle production in p+p and Pb+Pb collisions at the LHC, also study elliptic flow and  multiplicity fluctuations. These are  quantities that are essential for understanding the properties of the Quark Gluon Plasma, and the phase transition from normal matter  to quark gluon matter. Furthermore, the properties of particle emission at forward angles is sensitive to the properties of the Color Glass Condensate, a possible new state of matter in the colliding nuclei, which is a condensate of low momentum gluons and which could play the role of  Bose-Einstein condensate for strongly interacting system.

THE DETECTOR

The FMD consists of 5 rings of Si semiconductor detectors with a total of 51200 individual strips.

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The rings are of two types: the inner type consist of 10 wafers subdivided into 20 sectors with 1024 strips each. The outer type is subdivided into 40 sectors each with 512 strips. The Si wafers are 300 micrometer thick and are manufactured out of 6” diameter Si disks. The FMD consists of 3 groups of detectors called FM1, FMD2 and FMD3.  FMD 1 consist on a ring of inner type Si sensors. Both FMD2 and FMD3 consist of a ring of inner type Si sensors and a ring of outer type Si sensors.

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The FMD detector has been designed and constructed at the Niels Bohr Institute in Copenhagen and moved to CERN where it has been installed in the very center of ALICE on either side of the central region of ALICE.

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THE DATA FLOW

The signals for groups of 128 strips are preamplified and shaped in a VA chip (there are 400 VA chips in total), custom designed to withstand a total radiation exposure in excess of 200 krad. The signals from each VA chip are read out sequentially and fed into an ALTRO analog to digital chip, also used by other ALICE detectors. The ALTRO takes input from 16 VA preamplifier chips. There are 30 ALTRO chips in total which are mounted on 10 so-called FMD-digitizer cards, which are mounted right behind the detectors. The digitized  data from  FMD1, 2 and 3 are each read  by a Readout and Control Unit (RCU) and the data transferred via an optical link from the inside of ALICE to  the ALICE data acquisition system (DAQ). Here  data from the FMD is also merged with data from the other ALICE detectors on an event by event basis.

THE DATA AND THE DATA RECONSTRUCTION

For central (head-on) collisions of two lead nuclei we expect that all strips will be hit by charged particles in each event (on average between 0.5 and 3 charged particles per strip). The number of particles hitting each strip can be determined by measuring the total energy deposited by the particles in each strip an in each event and comparing that to the energy that a single particle deposits in its way through the detector strip.

For peripheral collisions or for proton-proton collisions the overall number of charged particles is much smaller (by about a factor of 200) and many strips will be empty, so that the number of charged particles in the acceptance of the detector can be determined by counting strips that have a ‘hit’.

To determine the real number of charged particles produced in the collisions, one must however first correct for the number of ‘secondary’ charged particles, i.e. particles that are produced when a ‘primary particle’, i.e. one produced in the collision hits a piece of material on the way to the detector.FMD Phytia & reconstructed

More information about the FMD:

Photos:

FMD3 on the way to the pit and installation...FMD3 on the way to the pit and installation...

FMD 3 installed in ALICE

FMD 3 installed in ALICE

Photo of FMD 2 in Lab

Photo of FMD 2 in Lab

More FMD 2

More FMD 2