ALICE实验：2.76 TeV Pb-Pb碰撞中直接光子椭圆流的观测

195 浏览量更新于2024-07-16 收藏 985KB PDF 举报

"在ALICE合作的一篇研究论文中，报告了在2.76 TeV的铅-铅(Pb-Pb)碰撞中，对于中间快度范围内的包容和直接光子的椭圆流(electric flow)进行了测量。研究集中在0-20%和20-40%的中心度类别，利用高度分段的电磁热量计(PHOS)以及通过检测器材料中的转换检测光子，并在中央跟踪系统中重建电子-正电子对(e+e-)。将这两种方法的数据结合，从而在横向动量范围0.9<pT<6.2 GeV/c内提取出直接光子的椭圆流。结果与RHIC（相对论重离子对撞机）的数据相比，显示出类似的直接光子椭圆流大小。虽然流体力学和运输模型的计算值低于实验数据，但被认为是相容的。" 这篇在《物理快报B》(Physics Letters B)上发表的研究，深入探讨了高能核碰撞中产生的椭圆流现象，这是理解夸克胶子等离子体(QGP)性质的关键指标之一。椭圆流是粒子在非均匀空间分布的初始状态下的集体运动效应，它反映了早期碰撞系统中的几何形状和动态演化。在这些高能碰撞中，产生的QGP是一种高温高密度的物质状态，其行为类似于理想液体。 ALICE实验利用高度分段的电磁热量计PHOS来探测光子，这是一种能精确测量光子能量和方向的设备。同时，通过检测器材料中的转化过程，即光子转化为电子-正电子对，再由中央跟踪系统重建这对粒子，提供了另一种探测手段。两种方法的结合使得测量更加精确，并且扩展了测量的动量范围。对比RHIC的测量结果，发现直接光子的椭圆流在两个不同的对撞机上具有相似的幅度，这可能意味着在不同能量尺度下，QGP的流动特性有某种共性。然而，流体力学和运输模型的预测值略低，这可能提示我们需要更精细的模型来更好地描述QGP的动力学，或者说明实验中观察到的椭圆流包含了一些模型中未考虑的机制。 ALICE实验的这一发现加深了我们对极端条件下QGP集体动力学的理解，并为改进理论模型提供了重要数据，这对于未来理解和模拟宇宙早期状态的物理过程具有重要意义。

310 ALICE Collaboration / Direct photon elliptic ﬂow in Pb–Pb

which is also known as cocktail simulation. The PCM and PHOS

measurements of inclusive photon ﬂow are performed indepen-

dently.

They are then combined and used with the combined ratio

as well as the calculated decay photon ﬂow.

The

photon elliptic ﬂow v

is calculated with the Scalar Prod-

uct

(SP) method, which is a two-particle correlation method [56],

using a pseudorapidity gap of |η| > 0.9 between the photon and

the reference ﬂow particles. The applied gap reduces correlations

not related to the event plane 

, such as the ones due to reso-

nance

decays and jets, known as non-ﬂow effects. The SP method

uses the Q -vector, computed from a set of reference ﬂow particles

(RFP) deﬁned as:





i∈RFP

inϕ

, (3)

where ϕ

is the azimuthal angle of the i-th RFP, n is the order of

the harmonic and w

is a weight applied for every RFP. The RFPs

are taken from the V0–A and V0–C detectors. Since these detectors

do not provide tracking information, we sum over the V0–A/V0–C

cells, while the amplitudes of the signal from each cell, which are

proportional to the number of particles that cause a hit, are used

as a weight w

. The non-uniformity of the detector azimuthal eﬃ-

ciency

is taken into account by applying the inverse of the event-

averaged

signal as a weight for each of the V0 segments, together

with a recentering procedure [50,57]. More speciﬁcally, the ellip-

tic

ﬂow v

is calculated using the unit ﬂow vector



= e

i2ϕ

built

from reconstructed photons











A∗





C∗





C∗



(4)

where the two pairs of brackets in the numerator indicate an av-

erage

over all photons and over all events; M

and M

are the

estimates of multiplicity from the V0–A and V0–C detectors, re-

spectively;

and



A∗



C∗

are the complex conjugates of the ﬂow

vector calculated in sub-event A and C, respectively.

In the PCM analysis, photons converting into e

−

pairs are

reconstructed with an algorithm which searches for displaced ver-

tices

with two oppositely charged daughter tracks. Only good

quality TPC tracks with a transverse momentum above 50 MeV/c

and

a pseudorapidity of |η| < 0.9are considered. The vertex

ﬁnding algorithm uses the Kalman ﬁlter technique for the de-

cay/conversion

point and four-momentum determination of the

neutral parent particle (V

) [58]. Further selection is performed

on the level of the reconstructed V

. Only V

s with a con-

version

points at radii between 5 < R < 180 cm are accepted

such that the π

and η-meson Dalitz decays are rejected and

to ensure a good coverage by the tracking detectors of the con-

version

daughters. To identify an e

−

pair, the speciﬁc energy

loss (dE/dx) in the TPC [50]of both daughters is used. The trans-

verse

momentum component q

of the electron momentum, p

with respect to the V

momentum-vector is restricted to be q

0.05



1 −

(

α/0.95

)

GeV/c, where α is the energy asymmetry of

the conversion daughters. Random associations of electrons and

positrons are further reduced by selecting V

s with cos(θ) > 0.85,

where θ is the pointing angle, which is the angle between the

momentum-vector of the e

−

pair and the vector that connects

the primary vertex and the conversion point. Based on the invari-

ant

mass of the e

−

pair and the pointing angle of the V

the primary vertex, the vertex ﬁnder calculates a χ

value which

reﬂects the level of consistency with the hypothesis that the V

Table 1

Summary

of the relative systematic uncertainties (in %) of the inclusive photon el-

liptic

ﬂow in the PCM and PHOS analysis, and of the decay photon simulation. All

contributions are expected to be correlated in p

with the magnitude of the relative

uncertainty varying point-by-point.

Centrality 0–20% 20–40%

(GeV/c) 2.0 5.0 2.0 5.0

PCM

Photon selection 2.4 4.2 2.1 4.0

Energy resolution 1.0 1.0 1.0 1.0

Eﬃciency 3 3 1.9 1.9

Total 4.0 5.3 3.0 4.5

PHOS

Eﬃciency & contamination 3.0 3.0 0.7 0.7

Event plane ﬂatness 2.0 2.0 1.4 1.4

Total 3.5 3.5 1.6 1.6

Decay photon calculation

Parameterization of v

1.3 3.6 0.8 2.2

η/π

normalization 1.7 3.2 1.7 2.4

Total 2.2 4.8 1.9 3.3

comes from a photon originating from the primary vertex. A selec-

tion

based on this χ

value is used to further reduce contamina-

tion

in the photon sample. The main sources of background that

remain after these selection criteria are V0s reconstructed from

∓

, π

∓

, π

∓

and e

∓

pairs, which is important to take

into account as shown in [59]. The elliptic ﬂow of this background

is subtracted using a side-band method approach. In this method,

the dE/dx information of both conversion daughters is combined

into a 1-dimensional quantity. The signal is a peaked distribution

and the side-bands are dominated by background sources. The v

of the side-bands is measured and subtracted from the main signal

region using the purity of the photon sample, which is obtained

by ﬁtting Monte Carlo templates to the data. The correction to the

measured inclusive photon ﬂow is of the order of 5% for central

and 2.5% for semi-central collisions, respectively.

The

systematic uncertainties of the inclusive photon ﬂow mea-

sured

with PCM are summarized in Table 1. The uncertainties

related to the photon selection (|η|, R, min p

, q

, χ

/ndf and

cos(θ)) are obtained by varying the selection criteria, and the sys-

tematic

uncertainties related to the contamination of the photon

sample are quantiﬁed by the uncertainty on the background ﬂow

subtraction. The energy resolution uncertainties, which are due to

detector resolution effects and bremsstrahlung of electrons, are

estimated by comparing v

γ ,inc

distributions as a function of the

reconstructed and true p

using MC simulations. The uncertainties

related to the variation of reconstruction eﬃciency in- and out-

of-plane

are calculated from studying the photon reconstruction

eﬃciency as a function of the track multiplicity. For most of these

sources only a small dependence on p

and collision centrality is

observed.

the PHOS analysis, the same photon selection criteria are ap-

plied

as in the direct-photon spectra analysis [23]. Cells with a

common edge with another cell that are both above the energy

threshold of 25 MeV are combined into clusters which are used as

photon candidates. To estimate the photon energy, the energies of

all cells or only those with centers within a radius R

core

= 3.5cm

from

the cluster center of gravity are summed. Compared to the

full cluster energy, the core energy is less sensitive to overlaps with

low-energy clusters in a high multiplicity environment, and is well

reproduced by GEANT3 Monte Carlo simulations [23]. The full en-

ergy

is used for the systematic uncertainty estimate. The contribu-

tion

of hadronic clusters is reduced by requiring E

cluster

> 0.3GeV,

cells

> 2 and by accepting only clusters above a minimum lat-

剩余14页未读，继续阅读

weixin_38747978

粉丝: 13
资源: 962

ALICE实验：2.76 TeV Pb-Pb碰撞中直接光子椭圆流的观测

sNN = 8.16 TeV时高多重性p + Pb碰撞中的魅力和奇异子强子的椭圆流

sNN = 2.76 TeV Pb–Pb碰撞中带电射流产生的方位各向异性

在2.76和5.02 A TeV Pb + Pb碰撞中的集体流

开关柜tev值骤增是发生了什么故障

纵向调制光学电压互感器和横向调制光学电压互感器优缺点

如何使用C51语言在Proteus中模拟AT89S51单片机控制LED显示特定图案的程序？请提供详细的步骤和示例代码。

上面的matlab代码补充完整，增加位置、墙体、植物蒸腾、土壤温度等参数

来自美强子衰变的电子的椭圆流是从$$ \ sqrt {s_ \ mathrm {NN}} = 2.76 \ \ hbox {TeV} $$ sNN = 2.76TeV的Pb–Pb碰撞数据中提取的

J / Ïˆ抑制sNN = 5.02 TeV下Pb–Pb碰撞中的前向速度

在sNN = 5.02TeV的Pb–Pb碰撞中快速测量Z 0玻色子产生

最新资源