When the LHC begins operation in 2008 the Forward Multiplicity Detector (FMD) at ALICE will measure charged particles at forward rapidities . The measurements of the event plane of the collisions will enable studies of the properties of in medium jet modifications. In this contribution the properties and capabilities of the ALICE FMD are discussed

Dalsgaard, H H

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PoS(LHC07)016The role of the FMD in high pT physicsHans H. Dalsgaard∗NBI, Copenhagen, DenmarkE-mail: hans.dalsgaard@cern.chWhen the LHC begins operation in 2008 the Forward Multiplicity Detector (FMD) at ALICEwill measure charged particles at forward rapidities . The measurements of the event plane of thecollisions will enable studies of the properties of in medium jet modifications. In this contributionthe properties and capabilities of the ALICE FMD are discussed.High-pT physics at LHCMarch 23-27, 2007University of Jyväskylä, Jyväskylä, Finland∗Speaker.c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/PoS(LHC07)016The role of the FMD in high pT physics Hans H. Dalsgaard1. Introduction: High pT physicsThe evolution of heavy ion collisions is often divided into two distinct momentum regimes.Low momentum particles (‘soft particles’) are well described by thermal models, whereas the highmomentum particles created as jets by direct interactions of the quarks can be described in theframework of pQCD (‘hard particles’). To study high pT particles, the nuclear modification factor,RAA is introduced [1]:RAA =d2NAA/dηd pTTAAd2σNN/dηd pTwith TAA = 〈Nbin/σ ppinelastic〉 (1.1)Related to this is the ratio of central to peripheral particle emission, simply defined as:RCP =Npd2Nc/dηd pTNcd2Np/dηd pT(1.2)If the heavy ion collisions scale with p+p collisions the value of RAA will be unity above somepT (around 1 GeV/c) as will RCP. This scaling was not found at RHIC. Figure 1 [2] shows RAAfrom Au+Au collisions at √sNN = 200 GeV measured by the BRAHMS experiment at RHIC. Thefigure clearly shows suppression of the nuclear modification factor as a function of pT , an effectknown as ’high pT suppression’. Another important study of jet behaviour in medium requires adetector with full azimuthal coverage. The idea is to study the jets with respect to the event plane.A measurement of this is shown in Figure 2. The jet with the longest in-medium path (the out-of-plane jet) is widened. The study shown in Figure 2 can be done by the ALICE experiment. For thisstudy the FMD can provide the measurement of the event plane independently of other detectors.2. ALICE and the FMDFigure 3 shows the ALICE [3] experiment. The heart of ALICE is the giant Time ProjectionChamber. The TPC, together with other tracking detectors and detectors for particle identification,provides tracking and PID in a pseudorapidity range of −0.9 < η < 0.9. The position of the FMD[4] partially inside the TPC is shown in Figure 3. The FMD can detect charged particles in thepseudorapidity ranges −3.4 < η <−1.7 and 1.7 < η < 5.4. A sketch of the FMD system is shownin Figure 4. Table 1 summarises the physical specifications of the FMD.Radial Coverage (cm) Azimuthal Sectors Radial StripsFMD1i 4.2-17.2 20 512FMD2i 4.2-17.2 20 512FMD2o 15.4-28.4 40 256FMD3i 4.2-17.2 20 512FMD3o 15.4-28.4 40 256Table 1: FMD segmentation and radial coverageThe FMD system consists of 5 rings each containing 10240 silicon strips for a total of 51200.There are three inner rings and two larger outer rings. Azimuthally the detector covers a full 2pi with2PoS(LHC07)016The role of the FMD in high pT physics Hans H. DalsgaardFigure 1: The nuclear modification factor measured by the BRAHMS experiment at RHIC. The lowerpanels show RCP [2].Ring z(cm) η coverageFMD1i 320 cm 3.68 < η < 5.03FMD2i 83.4 cm 2.28 < η < 3.68FMD2o 75.2 cm 1.70 < η < 2.29FMD3i -62.8 cm −3.40 < η <−2.01FMD3o -75.2 cm −2.29 < η <−1.70Table 2: FMD ring positions and pseudorapidity coverage.20 sectors in the inner rings and 40 sectors in the outer rings to allow for reasonable segmentationin φ . The FMD strips have good signal/noise ratio: better than 20:1 for the outer rings and betterthan 40:1 for the inner rings. A summary of the positions and coverage of the individual FMD ringsis shown in table 2.3. AnalysisThe purpose of the present study is to enable the reconstruction of data as soon as the LHC3PoS(LHC07)016The role of the FMD in high pT physics Hans H. DalsgaardFigure 2: Suppression of the out–of–plane jet measured by the STAR experiment at RHIC.begins operation. To facilitate this simulations are used to generate events and detector response.To generate√s = 14 TeV p+p events PYTHIA [5] is used and for generation of √sNN = 5.5 TeVPb+Pb events HIJING is used [6]. The ALICE geometry and the event generators are built intothe AliROOT software package used for the simulation and analysis. To simulate flow, a separatesoftware package has been used [7].The analysis proceeds strip-by-strip. The energy deposit of each strip is evaluated and a de-cision must be made for each strip whether or not to accept the strip as a hit or not. An algorithmhas been implemented to remove the effects of sharing where a particle deposits energy in severalstrips. Finally the multiplicity distribution is corrected for secondary particles, created in decaysand through interactions with the detector material and support structure.3.1 Charged Particle MultiplicityThe definition of the average charged particle multiplicity is simply dNdη , the number of particlesper unit of pseudorapidity per event. The reconstructed distribution of the multiplicity per eventin 100.000√s = 14 TeV p+p events is shown in Figure 5. Included in the plot is the primarydNdη , picked up from PYHTIA. As can be seen the reconstruction is successful as the reconstructedpoints and the input distribution are identical within the statistical errors.3.2 Event Plane and Elliptic FlowThe definitions of flow and the methods of analysing used here are based on [8]. The particle4PoS(LHC07)016The role of the FMD in high pT physics Hans H. DalsgaardFigure 3: The ALICE experiment.Figure 4: The Forward Multiplicity Detector. There are three inner and two outer rings. From left to rightare shown FMD 1,2,3. The interaction point is midway between FMD2 and FMD3.distributions can be written as the Fourier expansion:Ed3Nd3 p =12pid2NpT d pT dy(1+∞∑n=12vn cos(n(φ −Ψr))) (3.1)The components, vn =< cos(n(φ −Ψr)) > of this expansion characterise the flow. v1 is the directflow and v2 is the elliptical flow. The experimentally observed reaction plane, the event plane, Ψr5PoS(LHC07)016The role of the FMD in high pT physics Hans H. Dalsgaardη-5 0 5ηdN/d0510Figure 5: Reconstructed data (red points) plotted on top of the PYTHIA input function. The precision isgood, as the data match the input within statistical errors.is defined as:Ψn =1narctan( ∑i wi sin(nφi)∑i wi cos(nφi))(3.2)With these equations the event plane and the flow can be measured. There is an ambiguity in thisas the event plane to be used to calculate the vn should be calculated from particles not used forthe calculation of the coefficients themselves or else autocorrelations might occur. This defines oneof the roles of the FMD in ALICE; namely to provide the detectors at midrapidity with the eventplane angle. On the other hand the to calculate the vn in the FMD a way to avoid autocorrelationsis to use an event plane angle calculated by another detector in ALICE.In the following the effects of these autocorrelations have been ignored altogether. Figure 6shows the reconstructed second order event plane from 1000 Pb+Pb events. The event plane anglehas been reconstructed from a simulation where it was set to zero. The mean of the distributionwas found to be ψ2 = 0.033±0.12, consistent with this.Figure 7 shows the reconstructed v2 from 1000 Pb+Pb events. In the simulation v2 was set to0.05 and the result resembles this. A gaussian fit to the distribution gives v2 = 0.045±0.01 whichis in agreement with the value set in the simulations.4. ConclusionsHigh pT physics is an important topic of study at LHC. The ALICE experiment will be ableto do these studies at midrapidity. To do detailed studies of the in-medium modifications of jets the6PoS(LHC07)016The role of the FMD in high pT physics Hans H. Dalsgaard2Ψ-2 0 2Counts050100150200250Figure 6: The reconstructed event plane angle distribution fitted with a gaussian. The arrow indicates thevalue set in the simulation.FMD provides an independent precision measurement of the event plane. Furthermore the FMDprovides measurements of the charged particle multiplicity and the flow in Heavy Ion Collisions.References[1] John Adams et al. (STAR Collaboration) Experimental and theoretical challenges in the search for thequark gluon plasma: The star collaboration’s critical assessment of the evidence from RHICcollisions. Nucl. Phys., A757:102–183, 2005.[2] I. Arsene et al. (BRAHMS Collaboration) Quark gluon plasma and color glass condensate at RHIC?the perspective from the brahms experiment. Nucl. Phys., A757:1–27, 2005.[3] e. . Carminati, F. et al., “ALICE: Physics performance report, volume i,” J. Phys., vol. G30,pp. 1517–1763, 2004.[4] The ALICE Collaboration, “Alice Technical Design Report, Forward Detectors: FMD, T0, V0.”[5] Torbjorn Sjostrand et al. High-energy-physics event generation with pythia 6.1. Comput. Phys.Commun., 135:238–259, 2001.[6] Xin-Nian Wang and Miklos Gyulassy. Hijing: A monte carlo model for multiple jet production in p p,p a and a a collisions. Phys. Rev., D44:3501–3516, 1991.[7] Homepage of j.radomski: http://radomski.web.cern.ch/radomski/. World Wide Web.[8] Arthur M. Poskanzer and S. A. Voloshin. Methods for analyzing anisotropic flow in relativisticnuclear collisions. Phys. Rev., C58:1671–1678, 1998.7PoS(LHC07)016The role of the FMD in high pT physics Hans H. Dalsgaard2v-0.1 -0.05 0 0.05 0.1Counts01020304050Figure 7: The reconstructed values of v2 fitted with a gaussian. The arrow indicates the value set in thesimulation.8

The Role of the FMD in High pT Physics

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