5.02 TeV p-Pb碰撞中双射流kTy测量与核物质效应分析

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"这篇论文是ALICE合作组在《Physics Letters B》期刊上发表的一篇研究，详细介绍了在sNN = 5.02 TeV的质子-铅(p–Pb)碰撞中对双射流(dijet)横向动量(kT)的测量方法。研究使用了ALICE探测器，通过中央跟踪检测器上的带电粒子和电磁热量计中的中性能量来重建喷射。他们特别关注了双射流的相关性和核物质效应，这些效应可能在高能重离子碰撞中产生显著的影响。" 在高能物理领域，质子-铅碰撞是一种研究核介质效应的重要手段，它允许科学家们探索强相互作用下的基本粒子行为以及QCD（量子色动力学）在极端条件下的性质。在sNN = 5.02 TeV的能量下，ALICE合作组进行了详细的实验测量，分析了双射流的横向动量分布，这是理解喷射产生过程和核环境对粒子产生影响的关键参数。文章中提到的dijet kT是指两个关联喷流的横向动量之差，它可以通过测量带电和中性成分组成的全射流与仅由带电粒子组成的射流之间的方位角差异（Δφdijet）来确定。这种测量方法有助于揭示喷流在核物质中的偏振和不对称性，以及它们如何随事件多重性变化。文章指出，即使在不同事件多重性条件下，也没有发现明显的核物质效应导致的kTy变化，这与PYTHIA8模拟结果相对比，提供了对强相互作用理论的检验。 ALICE团队对每个事件进行逐事件校正，以消除底层事件（underlying event）的贡献，并且考虑了底层事件的波动和探测器的有限分辨率。这些校正确保了测量结果的准确性和可靠性。通过对dijet kTy的测量，研究者可以探究双射流的共面性，这是理解高能碰撞中喷流动力学的一个重要方面。这篇研究通过精确测量p-Pb碰撞中的双射流kT，提供了对核介质效应的新洞察，进一步推动了我们对强相互作用和高能重离子碰撞的理解。这一工作不仅对理论物理学家有重要意义，也为实验物理学家提供了关键的数据和分析方法。

ALICE Collaboration / Physics Letters B 746 (2015) 385–395 387

samples (see Section 2) only jets with p

ch+ne

,jet

> 40 GeV/c for the

low energy trigger and p

ch+ne

,jet

> 60 GeV/c for the higher energy

trigger are used. In these kinematic regimes the triggers are fully

eﬃcient and no fragmentation bias is observed with respect to

the minimum-bias jet sample. The |k

| distributions are reported

at particle level involving a correction for detector effects and

-smearing due to the underlying event, see Section 3.3.

The

dijet sample is biased due to the requirement that the full

jet has a larger transverse momentum than the associated charged

jet, while the full jet momentum is used to estimate k

. In an

unbiased measurement, the full jet would correspond to the lead-

ing

jet of the event in only 50% of the cases. A PYTHIA [29,30]

study

was performed in which the particle-level jet was deﬁned as

the jet containing all ﬁnal state particles (no kinematic selection

on constituents and full azimuthal acceptance). Applying the selec-

tion

of this analysis to detector-level reconstructed jets, results in

a correct tagging of the leading jet in 70% of the dijet events. This

results in a slightly harder |k

| distribution with a 10% smaller

yield at low |k

| and 20% higher yield at large |k

|. The results of

the p–Pb data analysis will be compared to particle-level PYTHIA

with the same dijet selection incorporating the mentioned bias.

The

dijet acoplanarity is measured as a function of the trans-

verse

momentum of the full jet while the kinematic interval of the

associated charged jet is also varied to explore k

for more or less

balanced dijets in transverse momentum. In addition k

distribu-

tions

are also presented for two event multiplicity classes.

3.3. Corrections and systematic uncertainties

The measured dijet |k

| distributions are corrected to the par-

ticle

level, deﬁned as the dijet |k

| from jets clustered from all

prompt particles produced in the collision including all decay

products, except those from weak decays of light ﬂavor hadrons

and muons. Both full and charged jets are accepted at particle

level in the full azimuthal acceptance and in the pseudorapid-

ity

range of |η

jet

| < 0.5. The correction to particle level is based

on a data-driven method to correct for the inﬂuence of the un-

derlying

event ﬂuctuations and on simulated PYTHIA events (tune

Perugia-2011 [31]) transported through the ALICE detectors layout

with GEANT3 [32]. The correction procedure takes into account the

and angular resolution of the measured dijets.

Detector-level

jets are deﬁned as jets reconstructed from recon-

structed

tracks and EMCal clusters after subtraction of the charged

energy deposits. The jet energy scale and resolution are affected

by unmeasured particles (predominantly K

and neutrons), ﬂuc-

tuations

of the energy deposit by charged tracks in the EMCal, the

EMCal energy scale and the charged particle tracking eﬃciency and

resolution. A response matrix as a function of p

T,jet

of the full

and associated charged jet, ϕ

dijet

and k

is created after match-

ing

the detector-level to the particle-level jets as described in [33].

The

-smearing due to ﬂuctuations of the underlying event

is estimated with the random-cones technique which is also ap-

plied

in the analysis of Pb–Pb data [24]. Cones with a radius equal

to the resolution parameter R are placed in the measured p–Pb

events at random positions in the η–ϕ plane ensuring the cone

is fully contained in the detector acceptance. The ﬂuctuations of

the background are characterized by the difference between the

summed p

of all the tracks and clusters in the random cone (RC)

and the estimated background: δ p



T,i

− A · ρ, where A is

the area of the random cone (A = π R

) and the subscript i indi-

cates

a cluster or track pointing inside the random cone. A random

cone can overlap with a jet but to avoid oversampling in small sys-

tems

like p–Pb, a partial exclusion of overlap with the leading jet

in the event is applied. This is achieved by excluding random cones

overlapping with a leading jet with a given probability, p = 1/N

coll

where N

coll

is the number of binary collisions. N

coll

is taken from

estimates applying a Glauber ﬁt to the multiplicity measured in

the V0A detector resulting in values between 14.7 and 1.52 de-

pending

on the event activity measured in the V0A detector. The

width of the background ﬂuctuations for full (charged) jets varies

between 2.12 (1.59) and 0.73 (0.56) GeV/c depending on the mul-

tiplicity

of the event.

The

inﬂuence of background ﬂuctuations is added to the re-

sponse

extracted from detector simulation through a Monte Carlo

model assuming that the background ﬂuctuation for the full and

associated charged jet are uncorrelated within 20% wide bins of

V0A multiplicity classes. Within these selected multiplicity classes

the variation of the background ﬂuctuations is negligible. Since the

Monte Carlo model does not generate full events and only accounts

for the δ p

smearing on a jet-by-jet basis, additional jet ﬁnding

ineﬃciencies and worsening of angular resolution due to the back-

ground

ﬂuctuation are not taken into account. These effects are

negligible since the contribution of the underlying event to the jets

in p–Pb collisions is small. No correction for the angular resolution

of the charged jet due to missing neutral fragments is applied. This

effect increases the width of ϕ

dijet

by ∼ 0.03 and is present, and

of the same magnitude, in the p–Pb data and the PYTHIA refer-

ence.

The

most probable correction to the jet energy, taking into ac-

count

detector effects and background ﬂuctuations, for fully recon-

structed

jets is 28% at p

ch+ne

,jet

= 20 GeV/c and decreases to 20%

for jets with p

ch+ne

,jet

> 40 GeV/c. The uncertainty on the jet energy

scale is evaluated by changing the tracking eﬃciency in data and

full detector simulation [34], varying the double counting correc-

tion

for the hadronic energy deposit in the EMCal and by using

different estimates of the underlying-event ﬂuctuations. The ﬁnal

uncertainty on the jet energy scale is 4%. The jet energy resolution

for full jets is 22% at p

ch+ne

,jet

= 20 GeV/c and decreases gradually to

18% at p

ch+ne

,jet

= 120 GeV/c. The inﬂuence of the uncertainties on

the jet energy scale and resolution on the dijet |k

| measurement

are discussed in the following.

The

measured |k

| distributions are corrected to the parti-

cle

level by applying bin-by-bin correction factors, which are

parametrized by a linear ﬁt to the ratio between the particle- and

detector-level |k

| distributions for a given dijet selection. The cor-

rection

factors take into account the effects of feed-in and feed-out

of the selected kinematic and angular intervals of the full and as-

sociated

charged jets. These effects slightly change the shape of

the |k

| distributions resulting in correction factors which vary

between 0.9for small |k

| to 1.2at large |k

|. The correction is

relatively small, because while feed-in from lower p

ch+ne

,jet

narrows

the |k

| distribution, feed-in from higher p

ch+ne

,jet

broadens the dis-

tribution

resulting in a cancellation. Similarly the feed-out to high

and low p

ch+ne

,jet

has a small effect on the observable. By using a

linear ﬁt to the correction factors the statistical ﬂuctuations of the

detector simulation are not propagated to the measurement. The

95% conﬁdence limit of the parametrization using the linear ﬁt is

included in the systematic uncertainty of the measurement. Cor-

rection

factors are extracted as a function of V0A event multiplicity

class and kinematic intervals of the full and associated charged jet.

The

dominant systematic uncertainty on the measurement orig-

inates

from the extraction of bin-by-bin correction factors. The un-

certainty

of the parametrization of the correction factors results in

10–20% correlated systematic uncertainty on the dijet |k

| yields.

An additional 2.5% uncertainty arises from the uncertainty on the

tracking eﬃciency which is 4%. Systematic uncertainties originat-

ing

from the charged hadron energy deposit in EMCal towers,

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