新方法：识别部分重建物体的喷流子结构标记器

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本文主要探讨了"用射流子结构标记部分重建的对象"这一主题，针对的是高能物理领域中的一个挑战，特别是在粒子物理学的顶夸克（top quark）衰变研究中。作者Marat Freytsis、Tomer Volansky和Jonathan R. Walsh提出了一个新的标记算法，其目标是识别那些只有部分成分被集中在一个单独喷射器（jet）中的不完整物体。这种现象在实验数据中可能会出现，比如在顶夸克衰变为W或b粒子时，如果W或b的某些衰变产物（如轻子或部分能量）并未完全被包含在顶夸克喷流（top jet）内，就会形成部分重建的情况。研究焦点放在了顶夸克衰变中的特殊现象，即强子衰变产生的W或b射流可能存在软弱部分，或者这些粒子的部分能量不在顶夸克射流锥（top jet cone）的典型边界内。为了识别这种残留的子结构，研究人员设计了一个可观察的指标，结合了射流子结构分析技术和积极的喷流清理技术，以减少量子色动力学（Quantum Chromodynamics, QCD）背景的干扰。这种方法特别适用于现有的标记器在某些“中间增强模式”下失效的情况，即射流结构难以精确解析的场景。此外，作者预计这个标记器不仅适用于识别标准模型中的非QCD强子喷射，例如来自隐藏山谷理论（Hidden Valley）预测的额外强子喷射，还可能扩展到其他需要识别不完整喷流结构的领域。论文发表于《物理学 letters B》（Physics Letters B, Volume 769, Issue 3, August 2017），并且开放获取，可供广大科研人员查阅和进一步研究。这项工作为高能物理实验提供了一种新颖且实用的方法，以增强对部分重建对象的识别能力，这对于理解和探测粒子物理过程中的细微细节具有重要意义。通过优化射流子结构分析策略，研究人员能够提高信号与背景分离的效率，从而推动前沿物理研究的进展。

Physics Letters B 769 (2017) 333–338

Contents lists available at ScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Tagging partially reconstructed objects with jet substructure

Marat Freytsis

a,∗

, Tomer Volansky

, Jonathan R. Walsh

c,d

Department of Physics, Harvard University, Cambridge, MA, 02138, USA

Raymond and Beverly Sackler School of Physics and Astronomy, Tel-Aviv University, Tel-Aviv 69978, Israel

Ernest Orlando Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720, USA

Berkeley Center for Theoretical Physics, University of California, Berkeley, CA 94720, USA

a r t i c l e i n f o a b s t r a c t

Article history:

Received

2 November 2015

Received

in revised form 14 July 2016

Accepted

3 August 2016

Available

online 24 August 2016

Editor: B.

Grinstein

Keywords:

QCD

jets

Hadronic

top decays

Jet

substructure

Top

tagging

We present a new tagger which aims at identifying partially reconstructed objects, in which only some of

the constituents are collected in a single jet. As an example, we focus on top decays in which either part

of the hadronically decaying W or the b jet is soft or falls outside of the top jet cone. We construct an

observable to identify remnant substructure from the decay and employ aggressive jet grooming to reject

QCD backgrounds. The tagger is complementary to existing ones and works well in the intermediate

boost regime where jet substructure techniques usually fail. It is anticipated that a similar tagger can be

used to identify non-QCD hadronic jets, such as those expected from hidden valleys.

(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP

1. Introduction

The successful discovery of the Higgs boson at the Large Hadron

Collider (LHC) [1,2] together with null results in new physics

searches stresses the need to improve and develop more sophis-

ticated

techniques to search for rare or exotic phenomena. Con-

siderations

related to the hierarchy problem typically predict new

physics which couples to the top quark and which is often charged

under QCD. As a consequence, such new physics may be buried

under immense hadronic background.

Interesting

classes of models that may escape detection are

ones that admit partially reconstructed objects. These occur when,

for example, constituents from the decay or shower of a heavy

particle are soft, not conﬁned to a single jet, or escape detection

altogether. Two examples are hadronic tops in which either part

of the hadronically decaying W or the b-quark falls outside of

the top-jet cone, and hidden valleys [3,4], in which some of the

hidden-sector particles are stable and escape detection while the

others decay to hadrons. In both cases, some of the information

on the hard parton which initiates the jet is missing and the jet

no longer has an obvious hard scale to distinguish it from QCD. As

a consequence it is hard to identify such events and with existing

tools new physics of this kind will go unnoticed.

Corresponding author.

E-mail

address: freytsis@uoregon.edu (M. Freytsis).

To improve on the situation, a better understanding of the

substructure of such jets is needed. In recent years, signiﬁcant

progress has been made in developing tools for studying the sub-

structure

of hadronic events in order to disentangle new physics

signals from background (see Ref. [5–7] and references therein).

Most existing techniques are effective when used on boosted ob-

jects,

and are therefore useful at discovering heavy new physics

particles that decay hadronically. On the other hand, partially re-

constructed

jets are best studied in the intermediate regime of

moderate boost. The production rates in this regime are often en-

hanced

by many orders of magnitude and hence may play a cru-

cial

role in discovering new physics. The available tools, however,

have signiﬁcantly reduced sensitivity in this region of parameter

space [8] despite its discovery potential. This letter focuses on

this moderate-boost region, demonstrating its utility by studying

partially reconstructed tops. The case of hidden valleys will be pre-

sented

elsewhere.

Top

quarks are a standard testing ground for boosted techniques

and are considered a standard candle for comparison between

methods [5,6]. The LHC experiments have used a variety of tech-

niques

to identify boosted top quarks, and used them to search

for new physics signals [10–16]. In the case of fully reconstructed

boosted tops, the jets typically admit 3 subjets and counting them

provides a powerful discrimination against QCD background. How-

ever,

when only two of the top decay products are reconstructed

as clear subjets, the resulting substructure exhibits only 2 subjets

and does not reconstruct the top. This sample is signiﬁcantly more

http://dx.doi.org/10.1016/j.physletb.2016.08.044

0370-2693/

SCOAP

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