利用LHC作为光子对撞机探索轻子与暗物质的新策略

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"利用LHC作为光子对撞机的轻子和暗物质搜索策略" 本文主要探讨了一种新的搜索策略，旨在利用大型强子对撞机（LHC）作为光子对撞机来探测标量轻子（sleptons）和暗物质粒子。这一策略的提出是基于当前对轻子质量以及μ子的磁矩（g-2）μ的理论与实验值之间存在的差异，这些差异暗示可能存在质量在15到60 GeV之间的轻子与质量比它高的中性暗物质粒子（χ01）。在传统的搜索方法中，探测轻子和暗物质粒子的尝试往往受到双玻色子背景噪音的困扰，这使得直接观测变得困难。作者Lydia Beresford和Jesse Liu提出了一种创新的解决方案，即通过在超外围碰撞中利用前向检测器来测量质子标记的初始状态动力学和缺失动量四矢量。这种方法可以有效地识别并减少不可还原的背景干扰。研究者展示了在13 TeV质子对撞机中，使用100 fb-1的碰撞数据，对于15 GeV到60 GeV质量差（Δm(˜l,˜χ01)）的轻子，可以达到200 GeV的质量上限，且探测敏感度超过了LEP（大型电子正电子对撞机）的实验结果。这一策略不仅提高了探测效率，还扩展了LHC在寻找新物理现象方面的探索范围。文章强调，利用LHC作为光子对撞机的这一策略有可能揭示出关于暗物质粒子性质的新信息，并可能对理解宇宙中的暗物质丰度提供关键线索。同时，这种技术的进步也为解决μ子的磁矩之谜提供了可能性，因为μ子的（g-2）μ值的差异可能是新物理学的证据。这篇开放访问的文章提出了一个创新的实验方案，利用LHC的独特能力，以更高效的方式探测轻子和暗物质，从而推动粒子物理学和宇宙学的研究。这一策略的成功实施将有助于拓宽我们对基本粒子性质和宇宙暗物质成分的理解，同时也为未来的实验设计和分析提供了有价值的指导。

Search Strategy for Sleptons and Dark Matter Using the LHC as a Photon Collider

Lydia Beresford

and Jesse Liu

†

Department of Physics, University of Oxford, Oxford OX1 3RH, United Kingdom

(Received 3 December 2018; revised manuscript received 22 July 2019; published 3 October 2019)

We propose a search strategy using the LHC as a photon collider to open sensitivity to scalar lepton

(slepton

l) production with masses around 15 to 60 GeV above that of neutralino dark matter

. This

region is favored by relic abundance and muon ðg − 2Þ

arguments. However, conventional searches are

hindered by the irreduc ible diboson background. We overcome this obstruction by measuring initial state

kinematics and the missing momentum four-vector in proton-tagged ultraperipheral collisions using

forward detectors. We demonstrate sensitivity beyond LEP for slepton masses of up to 200 GeV for

15 ≲ Δmð

Þ ≲ 60 GeV with 100 fb

−1

of 13 TeV proton collisions. We encourage the LHC

collaborations to open this forward frontier for discovering new physics.

DOI: 10.1103/PhysRevLett.123.141801

Introduction.—Elucidating the elementary properties of

dark matter (DM) is among the most urgent problems in

fundamental physics. The lightest neutralino

in super-

symmetric (SUSY) extensions of the standard model (SM)

is one of the most motivating DM candidates [1–3].A

favored scenario involves scalar partners of the charged

leptons (sleptons

l) being 1 to tens of GeV above the

mass. This enables interactions that reduce the

cosmo-

logical relic abundance to match the observed value [4]

via a mechanism called slepton coannihilation [5,6].

Furthermore, partners of the muon (smuon ˜μ) and neu-

tralinos with masses near the weak scale are a leading

explanation for 3σ to 4σ deviations between measurements

of the muon magnetic moment and SM prediction [7–10].

Remarkably, Large Hadron Collider (LHC) searches for

these key targets have no sensitivity when mass differences

are 15 ≲ Δmð

Þ ≲ 60 GeV [11–14]. Here, Large

Electron Positron (LEP) collider limits remain the most

stringent, excluding mð

lÞ ≲ 97 GeV [15–17]. Sensitivity

is hindered by an obstruction generic to all LHC search

strategies for invisible DM states and their mediators

[18–32]: the kinematics of colliding quarks and gluons

are immeasurable. Without this initial state information, the

missing momentum four-vector p

miss

left by DM can be

determined only in the plane transverse to the beam (p

miss

This precludes direct DM mass reconstruction that would

otherwise provide effective discrimination against neutrino

ν backgrounds.

This Letter proposes a search strategy to resolve these

long-standing problems by using the LHC as a photon

collider [33]. In a beam crossing, protons can undergo

an ultraperipheral collision (UPC), where photons from

the electromagnetic fields interact to produce sleptons

exclusively, pp → pðγγ →

lÞp. The sleptons decay as

l → l

, resulting in the very clean final state pð2l þ

miss

Þp of our search: two intact protons, two leptons l,

and missing momentum (Fig. 1). As the beam energy is

known, measuring the outgoing proton kinematics deter-

mines the colliding photon momenta and thus p

miss

. This

experimental possibility is opened by the ATLAS Forward

Proton (AFP) [34] and CMS-TOTEM Precision Proton

Spectrometer (CT-PPS) [35,36] forward detectors, which

recorded their first data in 2017 and 2016, respectively.

CMS-TOTEM moreover observed double lepton produc-

tion in high-luminosity proton-tagged events [37], demon-

strating that initial state reconstruction is feasible.

Photon collisions at the LHC reach sufficient rates to

probe rare processes such as SM light-by-light scattering

[38,39], anomalous gauge couplings [40,41], axionlike

particles [42,43], and dark sectors [44,45]. Nonetheless,

FIG. 1. Exclusive pair production of (left panel) scalar leptons

(“sleptons”)

l decaying to dark matter

and (right panel) SM

diboson WW background using the LHC as a photon collider.

Published by the American Physical Society under the terms of

the Creative Commons Attribution 4.0 International license.

Further distribution of this work must maintain attribution to

the author(s) and the published article’s title, journal citation,

and DOI. Funded by SCOAP

PHYSICAL REVIEW LETTERS 123, 141801 (2019)

0031-9007=19=123(14)=141801(7) 141801-1 Published by the American Physical Society

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