ALICE实验：5.02 TeV超外围p-Pb碰撞中J/ψ光子产生能区依赖性研究

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ALICE合作组织在一篇发布于《欧洲物理期刊C》（Eur.Phys.J.C）2019年第79卷的文章中，详细报告了他们在超外设p-Pb碰撞中对$$ \mathrm{J}/\psi $$矢量介子的光子产生能量依赖性的测量结果。这项研究是在质心能量为$$ \sqrt{s_{\mathrm{\scriptscriptstyle NN}}} = 5.02 $$ TeV的条件下进行的，即每个核子对的质心能量达到5.02 TeV。文章的DOI是10.1140/epjc/s10052-019-6816-2，并且具有开放访问的特性。在这项实验中，ALICE团队利用CERN的设施，通过检测$$ \mathrm{J}/\psi $$衰变成电子对($$ e^+e^- $$)和μ子对($$ \mu^+\mu^- $$)的事件，研究了$$ \mathrm{J}/\psi $$在负rapidity值(-2.5至2.7)范围内的光子产生截面与能量的关系。这个rapidity范围对应着γp质心系中的能量区间40<Wγp<550 GeV。研究结果显示，$$ \mathrm{J}/\psi $$的独家光子产生截面随着能量的增加遵循一个幂律依赖关系。这些测量结果与之前HERA和LHC的实验数据相一致，并且被用来对比和验证几种理论模型。分析表明，这些新测量的数据与先前的测量结果兼容，提供了对高能重离子碰撞中$$ \mathrm{J}/\psi $$产生机制更深入的理解，特别是在ultra-peripheral碰撞这一特殊环境下。在超外设碰撞中，由于相互作用的核子仅发生电磁相互作用，而不涉及强相互作用，这样的环境为研究高能光子与核子的相互作用提供了独特的机会。$$ \mathrm{J}/\psi $$作为重夸克对的束缚态，其产生过程能够揭示强子内部的夸克结构以及QCD（量子色动力学）在极端条件下的行为。 ALICE的这项工作不仅加深了我们对高能粒子碰撞的理解，也为未来的研究，比如在高能量下重离子碰撞中的夸克胶子等离子体（Quark-Gluon Plasma, QGP）性质探索，提供了宝贵的数据基础。此外，这些测量结果还可能对天体物理学中涉及高能宇宙线的光子与星际介质相互作用的研究有所启示。

402 Page 4 of 18 Eur. Phys. J. C (2019) 79 :402

To build dilepton candidates the tracks were separated into

electrons and muons using the energy loss measured by the

TPC, such that:

1. the quadratic sum of the normalised deviation from the

corresponding Bethe-Bloch expectation for the positive

and negative track had to be less than 16;

2. the rapidity, measured in the laboratory frame, of the

dilepton candidate, formed by the 4-vector sum of the

two tracks with the corresponding mass hypothesis, had

to be in the range |y| < 0.8.

In addition, the events had to

3. have no signal in V0 as determinedbythe ofﬂine decision,

and

4. no signal in either of the neutron ZDCs.

3.2 Event and track selection for the semi-backward and

semi-forward analyses

The events used in the semi-backward and semi-forward

analyses fulﬁlled the following criteria.

1. They ﬁred the corresponding trigger.

2. There was exactly one track reconstructed with the TPC

and the ITS.

3. This track fulﬁlled similar requirements to those for the

central analysis tracks, with two modiﬁcations: it had

to be compatible with a muon (within 4σ ) according to

the energy-loss measurement, and the acceptance was

enlarged to |η| < 1.1.

4. Furthermore, the distance of closest approach of thistrack

to the nominal interaction point had to be less than 15 cm

in the direction along the beam-line.

5. The event also had to have exactly one track in the muon

spectrometer such that:

(a) the track is matched to a trigger track above the p

threshold;

(b) the radial coordinate of the track at the end of the

front absorber (R

abs

) was required to be in the range

17.5 cm < R

abs

< 89.5 cm;

−2.5(−3.7 <η<−2.5) in the p–Pb (Pb–p) period

and fulﬁlled the so called p×DCA requirement.

The p×DCA requirement uses the difference in

the distributions for signal and beam-induced back-

ground in the ( p, DCA) plane, where p is the track

momentum and DCA is the distance between the

interaction vertex and the track extrapolated to the

vertex transverse plane. If in a given event no inter-

action vertex was found, the point (0, 0, 0) was used

to calculate the distance of closest approach. The

pseudorapidity ranges are different in p–Pb and Pb–p

owing to the additional V0C trigger requirement (see

Sect. 2.4).

6. The two tracks had to have opposite electric charge.

7. The rapidity, measured in the laboratory frame, of the

dimuon candidate, formed by the 4-vector sum of the two

tracks with the muon-mass hypothesis, had to be in the

range 1.2 < y < 2.7 for the p–Pb and −2.5 < y < −1.2

for the Pb–p sample, respectively.

8. In addition, the events had to have no signal in V0A, at

most one cell with a signal in V0C, (in both cases as

determined by the ofﬂine reconstruction), no signal in

either of the neutron ZDCs and at most one tracklet in

the SPD layers.

3.3 Selection of background samples

Several background samples, one for each analysis, were

selected. The events in these samples satisfy the condi-

tions listed in the previous sections, and in addition there

is required to be a signal, compatible with the presence of

a neutron, in the ZDC placed in the proton direction. Both

like-sign and opposite-sign dilepton candidates are accepted.

These samples are highly efﬁcient to tag dissociative pro-

cesses (where the target proton breaks up) and other hadronic

collisions and were used to build templates to subtract the

remaining background in the real data samples as explained

below. As the trigger suppresses events with higher p

dilep-

tons (p

≥ 0.5GeV/c), due to the condition on azimuthal

separation, the number of events in this p

range is limited.

Therefore, each background spectrum was used to gener-

ate a high statistics sample, which was smoothed to pro-

duce the ﬁnal template. This is referred to as “non-exclusive

background” in the analysis of the p

spectrum. The selec-

tions and smoothing algorithms were varied to estimate the

systematic uncertainties, and the background p

distribution

was veriﬁed independently by selecting hadronic activity in

V0C instead of the ZDC.

3.4 Monte Carlo samples

The STARLIGHT Monte Carlo generator [27,28] was used

to generate large samples for the following processes: exclu-

sive J/ψ photoproduction off proton (γ p → J/ψp) and

Pb (γ Pb → J/ψPb) targets, exclusive photoproduction

of ψ(2S) decaying into J/ψ + X, and continuum dilep-

ton production γγ → l

−

. The physics models used by

STARLIGHT are described in [28–30]. All generated events

were passed through a full simulation of the detector based

on GEANT 3 transport code [31] and subjected to the same

analysis chain as real data. The Monte Carlo tookintoaccount

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