LHC能区ALICE实验及其PHOS触发选判
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LHC能区ALICE实验及其PHOS触发选判. 王亚平 ( 华中师大 粒子物理研究所 ). 实验:超高能核 - 核碰撞. 4) Hadron gas freeze-out interacting hadronic matter. 0) initial conditions: target/projectile mass collision energy impact parameter. 1) hard interactions (jets, heavy quarks, J/ ).

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王亚平 ( 华中师大 粒子物理研究所 )

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LHC能区ALICE实验及其PHOS触发选判

王亚平

( 华中师大 粒子物理研究所 )


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实验:超高能核-核碰撞

4) Hadron gas

freeze-out

interacting hadronic matter

0) initial conditions:

target/projectile mass

collision energy

impact parameter

1)hard interactions

(jets, heavy quarks, J/)

3) quark-gluon plasma

Phase transition

formation of hadrons

2) pre-equilibrium

thermal system of partons


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Heavy ion实验@ BNL/RHIC(4km)

BRAHMS

PHOBOS

PHENIX

STAR

STAR 探测器

Heavy [email protected]/AGS

ALICE 探测器

Heavy ion实验@ CERN/LHC (27km)

LHC隧道

RHIC隧道

Heavy [email protected]/SPS (6.7km)


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碰撞的实验初始环境

expect conditions to be significantly different

Significant gain in ε, V, τ

»×10 SPS→ LHC

» ×3-5 RHIC →LHC

Central collisions

SPS

RHIC

LHC

s1/2(GeV)

17

200

5500

dNch/dy

430

700-1500

2-6×103

碰撞能量更高

碰撞核 更重

碰撞末态系统

-温度更热

-密度更密

–体积 更大

–寿命 更长

–产生粒子更多

ε(GeV/fm3)

2.5

3.5-7.5

15-40

Vf(fm3)

103

(?)7x103

2×104

τQGP (fm/c)

<1

1.5-4.0

4-10

τ0 (fm/c)

~1

~0.5

<0.2

LHC将于2007年正式投入物理运行

有关课题的理论研究和实验研究

更为必要,更为紧迫


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LHC

√s

[GeV]

RHIC

SPS

AGS

Ycm

Initial Conditions

pre-RHIC guess

still expect conditions to be significantly different


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RHICLHC

200 GeV 5500 GeV

ALICE @ LHC

CERN/LHC运行时将碰撞质心系能量为 =14 TeV的质子流和 =5.5 TeV的铅束流,包括四大实验:ATLAS,CMS,LHCb和ALICE。

ALICE(A Large Ion Collider Experiment),an experiment is optimized to study in detail the behaviour of matter at high densities and temperatures, in view of probing deconfinement and chiral-symmetry restoration。 ------探究QGP态物质


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HMPID

TOF

TRD

TPC

PMD

ITS

Muon Arm

ALICE Set-up

Size:16 x 26 meters

Weight:10,000 tons

PHOS


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108

106

104

102

100

LHC

M2 (GeV2)

RHIC

SPS

10-6 10-4 10-2 100

x

Novel aspects: Qualitatively new regime

  • Probe initial partonic state

    in a novel Bjorken-x range

    (10-3-10-5):

    • nuclear shadowing,

    • high-density saturated gluon distribution.

  • Parton dynamics dominate the fireball expansion


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(h++h-)/2

p0

5500 GeV

√s

=

200 GeV

17 GeV

LHC

RHIC

SPS

Novel aspects: Qualitatively new regime

  • Hard processes contribute significantly to the total AA cross-section

    (σhard/σtot = 98%)

    • Bulk properties dominated

      by hard processes;

    • Very hard probes are abundantly produced.

  • Weakly interacting probes become accessible

    (g, Z0, W±).


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ALICE/PHOton Spectrometer

PHOton Spectrometer (PHOS),will detect electromagnetic particles in a limited acceptance domain at central rapidity and provide photon identification as well as neutral mesons identification via the 2-photon decay channel.

Main physical goals:

Testing thermal and dynamical properties of the initial phase of collision, in particular the initial temperature and space-time dimensions of hot zone.

  • measurement of direct single-photon and di-photon spectra

  • measurement of Bose-Einsten correlations of direct photons

Investigating jet quenching as a probe of deconfinement, studied via high pTπ0 spectrum, and identifying jets through gamma-jet and jet-jet correlations measurements.

signals of chiral-symmetry restoration.


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Physical goals

  • Direct photons

  • once produced don’t interact with the fireball → carry information about early stage of a collision

  • Thermal photon spectra: pT ~ 5GeV

  • dominantly from early, hot QGP phase

     initial temperature of a QGP fireball

  • Thermal photon HBT correlations

     space-time dimension of the fireball and direct photon yield at low pT

  • Hard process (pQCD) photon spectra: pT>~ 5GeV

  • Test of Ncoll scaling of hard processes

  • γ– tagged jets

  • Spectra of identified π0 in a broad transverse momentum range

  • Determination of the decay photon background

  • Study of the jet quenching

  • Spectra of identified η-mesons


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Technical data:

17920 lead-tungstate crystals(PWO)

-distance to IP 4600mm

-coverage in pseudorapidity -0.12;+0.12

-coverage in azimuthal angle 100o

-crystal size 22x22x180 mm3

-depth in radiation length 20

-modularity 5 modules

-total area 8m2

-total crystal weight 12.5 t

-operating temperature -25 oC

-photon readout APD

PHOS (PHOton Spectrometer) is a high resolution electromagnetic calorimeter consisting of 17920 detection channels based on lead-tungstate crystals(PWO).


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PbW04 crystal

FEE card

in cooling cassette

Strip unit of 16 detectors

ALICE-PHOS前端电子学系统主板FEE

( 武汉组完成 )

Assembly of PHOS module


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Energy and spatial resolution of PHOS

从图中我们可以观察到在1 GeV/c时的位置分辨率依赖于角度的不同而从3.2 mm变化到3.9 mm,同样的在10 GeV/c时会从1.2 mm变化 到2.6 mm

能量分辨率从1 GeV/c时的4%变化到100 GeV/c时的1%


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Trigger system of ALICE

ALICE触发系统要解决的问题是如何充分利用各个子探测器在不同的碰撞模式下有效的进行触发判选。ALICE子探测器数据读出时间的不同要求ALICE触发系统有三个并行级别的触发,这样的设计使得ALCIE触发系统可以从较快的探测器中读出数据。ALICE触发系统的这三个级别的触发选判信号:零级触发(Level 0,简称L0),一级触发(Level 1,简称L1)和二级触发(Level 2,简称L2):

L0:固定延时,碰撞发生后1.2 μs产生,用来选通ALICE一些子探测器的前端电子学(FEE)部分。

L1:固定延时,碰撞发生后6.5 μs产生,选通L0触发之后余下的所有探测器,也被数据获取系统(DAQ)做为允许数据从探测器的前端电子学开始读出的触发信号。

L2:最终一级的触发信号,碰撞发生后88 μs后产生,主要目的是保证TPC探测器中不发生堆叠效应,同时参与测量的探测器的数据开始从各自的前端电子学部分读出并送到DAQ和HLT(High Level Trigger)系统。


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Trigger signals: L0, L1, L2

L0信号的产生主要基于T0和V0计数器,同时也依赖PHOS、EMCAL、前向TRD和μ子触发的输入。

L1是否触发是通过以下探测器的信息来判选的:(1)来自μ子探测器系统的信息;(2)来自中央探测器,如PHOS、FMD和ZDC的信息。

L2信号的产生基于TPC探测器的漂移时间。L2有可变的等待时间,其上限是TPC的漂移时间。

在ALICE触发系统中,有24个L0输入,20个L1输入和6个L2输入。

作为ALICE子探测器的PHOS( Photon Spectrometer )产生L0和L1触发信号,参与ALICE触发系统的工作。


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64 crystals ( 20 degree sector)

One TRU region

PHOS module

28

TRU

region

3584 Xtals

8 TRU domains

7 FEE cards

7 FEE cards

16

56 crystals

TRUcard

7 FEE cards

1 trigger region ( 1 TRU) = 448 crystals

1 Fast OR for 2*2 crystal signals

  • Fast OR generated per FEE

    112 Fast OR inputs to each TRU

Fast OR cables from FEE to TRU

Trigger decision criterion of PHOS

PHOS的触发逻辑在FPGA逻辑器件中得到执行,包括产生对应于高亮度下质子-质子碰撞中无偏碰撞事件的L0触发和对应于铅-铅碰撞中大横动量的光子事件的L1触发。


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58 Gbits/s permanent input!

Level-0

TRU

.

.

Level-1

112 inputs

FEE->TRU signal routing

112 analog sums are fanned in, via 14 FEE to 1 TRU

due to 2*2 sums one TRU covers 448 crystals

1 level-0 output

3 level-1 outputs


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40 MHz NRZ serial Trigger

Analog -> Digital

compare

Interaction

time t=0

40 MHz

FPGA

thresh.

FEE

40 MHz

TOF shower APD analog Sum

NRZ

12 bit

ADC

130ns

peak

Over

Samp

15 ns

20ns 20ns 10 ns 80 ns

4-deep time summing

25 ns

25 ns

25 ns

25 ns

4*4

space

Sum

25ns

CTP

DeSer

25ns

160 ns

pipeline

~ 40 m = 200 ns

~ 300 ns

~ 350 ns

~ 400 ns

600 ns out of FPGA

max. 800 ns (L0)

fixed delay

Trigger decision criterion of PHOS

ALICE实验运行时,允许PHOS产生L0触发信号的等待时间只有800 ns,产生L1触发信号的等待时间只有6200 ns。

PHOS采用TRU单元及其移动窗算法产生触发信号L0或L1,大幅度的缩短PHOS产生触发信号的时间。


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Energy reconstruction performance

高斯分布的平均值μ=5.510 GeV,方差σ=0.104 GeV,σE/E=1.89%;

高斯分布的平均值μ=5.660 GeV,方差σ=0.099 GeV,σE/E=1.75%。

在大横动量范围里PHOS对电子的重建能量与电子的入射能量表现出理想的线性关系


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Trigger efficiency of PHOS

Trigger threshold

触发效率分布满足方程:

Trigger efficiency loss about ~ 4% due to the TRU boundaries


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1 module

3 modules

5 modules

Trigger rate of PHOS for P-P collision

●pp run: L = 5×1030 cm-2s-1, √sNN=14 TeV, mini-bias events σtotal~0.1 b; Pythia6.2, 6×106 pp collision events.


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1 module

3 modules

5 modules

Trigger rate of PHOS for Pb-Pb collision

● PbPb run: L = 5×1026 cm-2s-1, √sNN=5.5 TeV,σtotal~7.7 b; Hijing1.36, 1×105 PbPb collision events, impact parameters (0, 3 fm).


Summary

Summary

○LHC/ALICE is a powerful next generation detector

  • first truly general purpose HI experiment

    • addresses most relevant observables: from super-soft to ultra-hard

  • many evolutionary developments: SSD, SDD, TPC, em cal, …

  • some big advances in technology:electronics, pixels, TOF, computing

○ALICE/PHOS performs well in trigger decision

  • with good performance of energy reconstructionin abroad transverse momentum range from 0.5 GeV/c to 100 GeV/c

  • trigger efficiency: 1) showed a first order exponential decay versus pT, 2) showed a loss about 4% due to the TRU boundaries

  • trigger rate: 1) for direct photons was much higher than it for π0and η-mesons, 2) PHOS could provide L0 trigger by its FEE card to low pTphotonevents and ALICE/CTP provide L0 trigger to high pTphotonevents, 3) PHOS could provide L1 trigger by its FEE card.


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Thanks!


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PHOS mechanics

Crystal detector unit

PHOS module

Working temperature: -25 oC

Modular structure

  • 5 independentmoduleseachof 3584crystal detector units:

  • PWO crystal+ APD+ preamp.

Strip unit of 16 detector units

PHOS Cradle


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