ATLAS探测器分析8 TeV t通道单顶夸克产生中的Wtb顶点

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"这篇论文是ATLAS合作组发布的一篇开放获取文章，详细研究了在质子-质子碰撞中，通过t通道产生的单个顶夸克的三微分角衰减率，以此来分析Wtb顶点的性质。文章发表在JHEP12(2017)017上，由Springer出版，2017年7月19日收到，11月21日接受，12月4日正式发表。" 在高能物理领域，顶夸克是已知最重的基本粒子之一，它的产生和衰变过程能够揭示基本粒子相互作用的细节。t通道中的顶夸克电弱产生和衰变与Wtb顶点密切相关，这个顶点涉及W玻色子与顶夸克和b夸克之间的相互作用。Wtb顶点可以通过有效的拉格朗日量参数来描述，这些参数具有复数特性。本文对t通道中的三微分衰减率进行了分析，这种方法允许同时确定五个广义螺旋度分数和相位，以及生成的顶夸克的极化状态。分析基于ATLAS探测器在大型强子对撞机（LHC）上收集的20.2 fb^{-1}的质子-质子碰撞数据，数据集的质心能量为8 TeV。测量结果显示，包含横向极化的W玻色子的衰变分数（f_1）为0.30±0.05。此外，测得相对于左旋b夸克，横向和纵向极化的W玻色子反冲的振幅之间的相位（δ^-）为0.002π+ 0.017π+ 0.016π，未观察到违反CP对称性的迹象。同时，右旋b夸克伴随的纵向或横向W玻色子的分数也得到了限制。基于这些测量结果，研究者对复数耦合参数Re[g_R / V_L]和Im[g_R / V_L]的比值设定了95%置信水平（CL）的限制，其范围分别为[-0.12，0.17]和[-0.07，0.06]。此外，还对|V_R / V_L|和|g_L / V_L|的比率进行了约束。单个顶夸克在t通道中的极化也被限制为P> 0.72（95% CL）。这些测量结果均独立于其他参数或耦合值，提供了一种全面理解Wtb顶点特性的方法，对于深入理解标准模型以及可能的新物理现象具有重要意义。

JHEP12(2017)017

to the IP, surrounded by a superconducting solenoid providing a 2 T axial magnetic ﬁeld,

electromagnetic (EM) and hadronic calorimeters, and a muon spectrometer (MS). The

ID consists of a silicon pixel detector, a silicon micro-strip detector, providing tracking

information within pseudorapidity |η| < 2.5, and a straw-tube transition radiation tracker

that covers |η| < 2.0. The central EM calorimeter is a lead and liquid-argon (LAr) sampling

calorimeter with high granularity, and is divided into a barrel region that covers |η| < 1.5

and endcap regions that cover 1.4 < |η| < 3.2. A steel/scintillator tile calorimeter provides

hadronic energy measurements in the central range of |η| < 1.7. The endcap (1.5 <

|η| < 3.2) and forward regions (3.1 < |η| < 4.9) are instrumented with LAr calorimeters

for both the EM and hadronic energy measurements. The MS consists of three large

superconducting toroid magnets with eight coils each, a system of trigger chambers covering

|η| < 2.4, and precision tracking chambers covering |η| < 2.7. The ATLAS detector employs

a three-level trigger system [45], used to select events to be recorded for oﬄine analysis.

The ﬁrst-level trigger is hardware-based, implemented in custom-built electronics and it

uses a subset of the detector information to reduce the physical event rate from 40 MHz to

at most 75 kHz. The second-level trigger and the ﬁnal event ﬁlter, collectively referred to

as the high-level trigger (HLT), are software-based and together reduce the event rate to

about 400 Hz.

4 Data and simulation samples

The analysis is performed using data from pp collisions delivered by the LHC in 2012 at

√

s = 8 TeV and recorded by the ATLAS detector. Stringent detector and data quality

requirements were applied, resulting in a data sample corresponding to a total integrated

luminosity of 20.2 fb

−1

[46]. The events were selected by single-lepton

triggers [45, 47],

imposing at the HLT a threshold of 24 GeV on the transverse energy (E

) of electrons and

on the transverse momentum (p

) of muons, along with isolation requirements. To recover

eﬃciency for higher-p

leptons, the isolated lepton triggers were complemented by triggers

without isolation requirements, but with a threshold raised to 60 GeV for electrons and to

36 GeV for muons.

Samples of events generated using Monte Carlo (MC) simulations were produced using

diﬀerent event generators interfaced to various parton showering (PS) and hadronisation

generators. Minimum-bias events simulated with the Pythia8 generator (ver. 8.1) [48]

were overlaid to model the eﬀect of multiple pp collisions per bunch crossing (pile-up). The

distribution of the average number of pile-up interactions in the simulation is reweighted to

match the corresponding distribution in data, which has an average of 21 [46]. The events

were processed using the same reconstruction and analysis chain as for data events.

Single-top-quark t-channel events were generated with the NLO Powheg-Box gen-

erator (rev. 2556) [49] with the CT10f4 [50] PDF set, using the 4FS for the matrix-

element (ME) calculations [51]. The renormalisation and factorisation scales were set to

= µ

= 16(m

+ p

T,b

), where m

is the mass of the b-quark and p

T,b

is the transverse

momentum of the b-quark from the initial gluon splitting. Top quarks were decayed using

Henceforth, “lepton” indicates electron or muon, and does not include τ leptons.

– 9 –

JHEP12(2017)017

MadSpin [52], which preserves all spin correlations. Additional t-channel samples were

produced with the LO Protos generator (ver. 2.2b) [53] using the CTEQ6L1 PDF set [54]

within the 4FS. Thus in addition to a SM sample, samples with anomalous couplings en-

abled in both the production and the decay vertices were produced using the Protos gen-

erator, varying simultaneously V

with either Re [V

] ∈ [0.25, 0.50], Re [g

] ∈ [−0.26, 0.18]

or Im [g

] ∈ [−0.23, 0.23], such that the top-quark width was invariant. The factorisation

scale was set to µ

= −p

for the spectator quark and µ

= p

+ m

for the gluon, where

and p

are the three-momenta of the exchanged W boson and of the

b-quark origi-

nating from the gluon splitting (the spectator

b-quark), respectively. In order to compare

diﬀerent LO generators, another sample of signal events was produced with the multi-leg

LO AcerMC generator (ver. 3.8) [55] using the CTEQ6L1 PDF set. This generator in-

corporates both 4FS and 5FS, featuring an automated procedure to remove the overlap in

phase space between the two schemes [56]. The factorisation and renormalisation scales

were set to µ

= µ

= m

= 172.5 GeV.

In this analysis, all simulated signal event samples are normalised using the production

cross-section mentioned in section 1. Simulation samples produced with Powheg-Box

are used for predicting the acceptance and the template shape of the t-channel signal. To

estimate the eﬃciency and resolution models, the simulation samples in which parton-level

information is well deﬁned, i.e. those produced with either Protos or AcerMC, are used.

Samples of simulated events for t

t production and electroweak production of single

top quarks in the associated Wt and s-channel were produced using the NLO Powheg-

Box generator (rev. 2819, rev. 3026) coupled with the CT10 [50] PDF set. The t- and

s-channel processes do not interfere even at NLO in QCD and are thus well deﬁned with

that precision [57]. For Wt associated production, the diagram removal scheme is used to

eliminate overlaps between this process and t

t production at NLO. In the t

t sample, the

resummation damping factor

damp

was set to the top-quark mass [58]. An additional

t sample with anomalous couplings enabled in the decay vertex was produced using the

Protos generator (ver. 2.2) coupled with the CTEQ6L1 PDF set. This sample is used

to take into account the dependence of t

t background upon the value of the anomalous

couplings.

For all simulated event samples mentioned above, the PS, hadronisation and underlying

event (UE) were added using Pythia (ver. 6.426, ver. 6.427) [59] with the Perugia 2011C

set of tuned parameters (P2011C tune) [60] and the CTEQ6L1 PDF set. The Tauola [61]

program and the Photos [62] algorithm were used to properly simulate decays of polarised

τ leptons including spin correlations and to generate quantum electrodynamics (QED)

radiative corrections in decays to account for photon radiation. All these processes were

simulated assuming a top-quark mass of 172.5 GeV, and the decay of the top quark was

assumed to be 100% t → W b.

For estimating the t-channel and t

t generator modelling uncertainties, additional sam-

ples were produced using alternative generators or parameter variations. For studying the

The resummation damping factor, h

damp

, is one of the parameters controlling the ME/PS matching

in Powheg and eﬀectively regulates the high-p

gluon radiation. In the used Powheg-Box revision,

damp

= ∞ was the default value.

– 10 –

JHEP12(2017)017

top-quark mass dependence, supplementary single-top-quark and t

t simulated event sam-

ples with diﬀerent top-quark masses were generated. These topics are further discussed in

section 10 and section 11, respectively.

Vector-boson production in association with jets was simulated using the multi-leg LO

Sherpa generator (ver. 1.4.1) [63] with its own parameter tune and the CT10 PDF set.

Thus, W+jets and Z+jets events with up to four additional partons were generated and the

contributions of W/Z+light-jets and W/Z+heavy-jets (W/Z+bb, W/Z+cc, W/Z+c) were

simulated separately. Sherpa was also used to generate the hard process, but also for the

PS, hadronisation and the UE, using the CKKW method [64] to remove overlaps between

the partonic conﬁgurations generated by the ME and by the PS. Samples of diboson events

(W W , W Z, and ZZ), containing up to three additional partons where at least one of the

bosons decays leptonically, were also produced using the Sherpa generator (ver. 1.4.1)

with the CT10 PDF set.

All baseline simulated event samples were passed through the full simulation of the

ATLAS detector [65] based on the GEANT4 framework [66] while Protos simulated event

samples and alternative samples used to estimate systematic uncertainties were processed

through a faster simulation using the Atlfast2 framework [67].

5 Event reconstruction

Electron candidates are reconstructed from isolated energy deposits in the EM calorimeter

associated with ID tracks fulﬁlling strict quality requirements [68]. These electrons are

required to satisfy E

= E

cluster

/ sin(θ

track

) > 25 GeV and |η

cluster

| < 2.47, where E

cluster

and η

cluster

denote the energy and the pseudorapidity of the cluster of energy deposits in

the EM calorimeter, and θ

track

denotes the polar angle of the ID track associated with this

cluster. Clusters in the EM calorimeter barrel-endcap transition region, corresponding to

1.37 < |η

cluster

| < 1.52, are excluded. Muon candidates are reconstructed using combined

information from the ID tracks and the MS [69]. They are required to have p

> 25 GeV

and |η| < 2.5. The electron and muon candidates must fulﬁl additional isolation require-

ments, as described in ref. [70], in order to reduce contributions from misidentiﬁed jets,

non-prompt leptons from the decay of heavy-ﬂavour quarks and non-prompt electrons from

photon conversions.

Jets are reconstructed using the anti-k

algorithm [71, 72] with a radius parameter

of 0.4, using topological clusters of calorimeter energy deposits [73] as inputs to the jet

ﬁnding. The clusters are calibrated with a local cluster weighting method [73]. The jet

energy is further corrected for the eﬀect of multiple pp interactions. Jets are calibrated

using an energy- and η-dependent simulation-based scheme, with in situ corrections based

on data [74]. To reject jets from pile-up events, a so-called jet-vertex-fraction (JVF) crite-

rion [75] is applied to the jets with p

< 50 GeV and |η| < 2.4: at least 50% of the scalar

sum of the p

of the tracks associated with a jet is required to be from tracks compatible

with the primary vertex.

Only events containing reconstructed jets with p

> 30 GeV and

A primary-vertex candidate is deﬁned as a reconstructed vertex with at least ﬁve associated tracks with

> 400 MeV. The primary vertex associated with the hard-scattering collision is the candidate with the

largest sum of the squared p

of the associated tracks.

– 11 –

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