ALICE合作：5.02 TeV超外围Pb-Pb碰撞中相干J / ψ光产生的初步测量

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ALICE合作组在TeV能量尺度上对超外围Pb-Pb碰撞中的相干J /ψ光产生进行了首次速度差异测量。在5.02 TeV的质量能量sNN下，他们在正向快速区域(-4.0 < y < -2.5)通过Dimuon衰变检测到了J /ψ。分析利用了大约750μb^-1的综合亮度对应的事件样本。正文: 在高能粒子物理学领域，ALICE（大型离子对撞机实验）合作组在5.02 TeV的质心能量下对超外围Pb-Pb碰撞进行了深入研究，这些碰撞是一种特殊类型的核碰撞，其中两个铅核几乎不发生物质交换，只通过电磁相互作用。在这样的碰撞中，产生了大量的光子，这些光子可以与核内的虚拟胶子相互作用，从而导致所谓的相干J /ψ光产生。J /ψ是由一个重夸克对（charm quark和anticharm quark）组成的束缚态，其衰变通常表现为μ+μ-对。这次测量的关键在于对正电子活动度的要求，即选择那些hadronic活动最小的事件，这样可以减少非相干过程的干扰，更准确地探测到相干J /ψ光产生的信号。在正向快速区域，也就是远离质心系统的区域，J /ψ通过Dimuon衰变被检测到，这是一个标准的J /ψ探测技术。分析结果基于大约750微barn倒数（μb^-1）的综合亮度，这代表了相当大的统计样本，使得测量具有很高的精度。将测量的相干J /ψ产生的横截面与六个速度仓（rapidity bins）的数据对比，可以提供关于这个过程的详细信息。横截面是描述粒子产生概率的重要物理量，它随rapidity的变化提供了关于J /ψ产生的动力学信息。这些测量结果被用来与各种理论模型进行比较，以理解胶子阴影效应的影响。胶子阴影效应是指在高密度介质中，由于胶子的相互作用，胶子的有效分布函数会受到抑制，这在高能量核碰撞中是重要的考虑因素。比较表明，胶子阴影效应在J /ψ的光生产过程中确实发挥了作用。此外，还测量了ψ'（另一个重夸克对的束缚态）与J /ψ相干光产生截面的比率。这个比率的测量与在质子上的光产生测量结果一致，这为进一步理解光产生过程中的基本物理机制提供了宝贵线索。这些研究不仅有助于深化我们对强相互作用的理解，也为未来更高级别的实验，如在更高等能量的对撞机上的实验，奠定了基础。总结来说，ALICE合作组的这项工作揭示了在超外围Pb-Pb碰撞中相干J /ψ光产生的特性，并通过与理论模型的比较，确认了胶子阴影效应的存在。这一发现对于理解核物质中的量子色动力学（QCD）效应，特别是在极端条件下，具有重要意义。

ALICE Collaboration / Physics Letters B 798 (2019) 134926 3

Fig. 1. Left: invariant mass distribution for muon pairs satisfying the event selection described in the text. The dashed green line corresponds to the background. The solid

magenta and red lines correspond to Crystal Ball functions representing J/ψ and ψ



signals, respectively. The solid blue line corresponds to the sum of background and signal

functions. Right: transverse momentum distribution for muon pairs in the range 2.85 < m

μμ

< 3.35 GeV/c

(around the J/ψ mass).

−4.0 < y < −2.5 and in Fig. 2 in six rapidity subranges. The in-

variant

mass distributions are ﬁtted with a function modeling the

background and two Crystal Ball functions [26]for the J/ψ and the



peaks. The shape of the background at large invariant masses is

well described by an exponential distribution, as expected if it is

dominated by the process γγ → μ

−

. However, at masses be-

low

the J/ψ , the distribution is strongly inﬂuenced by the muon

trigger condition. In order to model this, the whole background

distribution is ﬁtted using a template made from reconstructed

STARlight events corresponding to the γγ → μ

−

process. The

results of the ﬁt are parametrized using a fourth-order polynomial,

which turns smoothly into an exponential tail as from 4GeV/c

The coeﬃcients of the polynomial are then kept ﬁxed in the ﬁt to

the experimental data, while the slope of the exponential term and

the normalization are left free. The ﬁtted slope is found to agree

within 2.5 standard deviations with the value obtained from the

generated sample.

The

raw inclusive J/ψ and ψ



yields, N(J/ψ) and N(ψ



), were

obtained by ﬁtting the dimuon invariant mass spectrum in the

range 2.2 < m

μμ

< 6GeV/c

. The slope parameters in the Crys-

tal

Ball functions were ﬁxed from ﬁts to the respective Monte

Carlo sets. The width parameter σ

J/ψ

was left free for the J/ψ ,

and was ﬁxed to σ



= σ

J/ψ

· (σ



/σ

/ψ

) for the ψ



, where the

ratio σ



/σ

/ψ

∼ 1.09 of the ψ



to the J/ψ widths was obtained

from the ﬁts to corresponding Monte Carlo sets. The mass parame-

ter

of the Crystal Ball function was left unconstrained for the J/ψ.

Due to the small ψ



statistics, the ψ



mass was ﬁxed so that the

difference with respect to the J/ψ mass is the same as quoted by

the PDG [27]. The J/ψ mass m

J/ψ

= 3.0993 ± 0.0009 GeV/c

, ob-

tained

from the ﬁt in the full rapidity range −4.0 < y < −2.5, is

in agreement with the PDG value within 3 standard deviations.

The

raw inclusive J/ψ yields obtained from invariant mass ﬁts

contain contributions from the coherent and incoherent J/ψ pho-

toproduction,

which can be separated in the analysis of transverse

momentum spectra. The p

distributions for dimuons in the range

2.85 < m

μμ

< 3.35 GeV/c

are shown in Fig. 1, right, in the full

dimuon rapidity range −4.0 < y < −2.5 and in Fig. 3 in six ra-

pidity

subranges. These distributions were ﬁtted with Monte Carlo

templates produced using STARlight, corresponding to different

production mechanisms: coherent J/ψ , incoherent J/ψ, feed-down

J/ψ from coherent ψ



decays, feed-down J/ψ from incoherent ψ



decays and continuum dimuons from the γγ →μ

−

process. In

order to describe the high-p

tail, the incoherent J/ψ photopro-

duction

accompanied by nucleon dissociation was also taken into

account in the ﬁts with the template based on the H1 parametriza-

tion

of the dissociative J/ψ photoproduction [28](denoted as dis-

sociative

J/ψ in the following):

∼ p



1 +



−n

. (1)

The H1 collaboration provided two sets of measurements corre-

sponding

to different photon–proton center-of-mass energy ranges:

25 GeV < W

γ p

< 80 GeV (low-energy data set) and 40 GeV <

γ p

< 110 GeV (high-energy data set). The ﬁt parameters b

1.79 ±0.12 (GeV/c)

−2

and n

= 3.58 ±0.15 from the high-energy

data set were used by default, while the corresponding uncer-

tainties

and the low-energy values b

= 1.6 ± 0.2 (GeV/c)

−2

and

= 3.58(ﬁxed) were used for systematic checks.

The templates were ﬁtted to the data leaving the normaliza-

tion

free for coherent J/ψ , incoherent J/ψ and dissociative J/ψ

production. The normalization of the γγ → μ

−

spectrum was

ﬁxed to the one obtained from the invariant mass ﬁts. The nor-

malization

of the coherent and incoherent feed-down J/ψ tem-

plates

was constrained to the normalization of primary coherent

and incoherent J/ψ templates, according to the feed-down frac-

tions

extracted from the measurement of raw inclusive J/ψ and



yields, as described below. The extracted incoherent J/ψ frac-

tion

N(incoh J/ψ)

N(coh J/ψ)

for p

< 0.25 GeV/c ranges from (4.9 ± 0.6)%

to (6.4 ± 0.8)% depending on the rapidity interval and is consis-

tent

with being constant within the uncertainties of the ﬁts. The

contribution of incoherent J/ψ with nucleon dissociation was also

taken into account in this fraction.

4. Results and discussion

4.1. Ratio of coherent ψ



and J/ψ cross sections

The obtained dimuon invariant mass spectra can be used to ex-

tract

the ratio of coherent ψ



and J/ψ cross sections R =

σ (ψ



)

σ (J/ψ )

and the fraction of feed-down J/ψ from ψ



decays in the raw J/ψ

yields. The ﬁts to the invariant mass distributions for dimuons with

pair p

< 0.25 GeV/c in the full rapidity range −4.0 < y < −2.5

result

in the following ratio of the measured raw inclusive ψ



and

J/ψ yields:

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