ATLAS观察到光与光散射的直接证据

诸平

Figure 1: Diagrams illustrating the QED LbyL interaction processes and the equivalent photon approximation.

a, Diagrams for Delbrück scattering (left), photon splitting (middle) and elastic LbyL scattering (right). Each cross denotes external field legs, for example, an atomic Coulomb field or a strong background magnetic field. b, Illustration of an ultra-peripheral collision of two lead ions. Electromagnetic interaction between the ions can be described as an exchange of photons that can couple to form a given final state X. The flux of photons is determined from the Fourier transform of the electromagnetic field of the ion, taking into account the nuclear electromagnetic form factors.

a, Photon PID efficiency as a function of photon ET extracted from FSR event candidates. b, Photon reconstruction efficiency as a function of photon ET (approximated with ET^e − pT^trk2) extracted from γγ e⁺e⁻ events with a hard-bremsstrahlung photon. Data (filled markers) are compared with MC simulations (open markers). The statistical uncertainties arising from the finite size of the data and simulation samples are indicated by vertical bars.

Figure 3: Kinematic distributions for γγ γγ event candidates.

a, Diphoton acoplanarity before applying the Aco < 0.01 requirement. b, Diphoton invariant mass after applying the Aco < 0.01 requirement. Data (points) are compared to MC predictions (histograms). The statistical uncertainties on the data are shown as vertical bars.

据欧洲核子研究中心（CERN）2017年8月15日提供的消息,ATLAS观察到光与光散射的直接证据。ATLAS是大型强子对撞机(Large Hadron Collider简称LHC)的两个通用探测器之一，详见下面照片。

A light-by-light scattering event measured in the ATLAS detector. Credit: ATLAS/CERN

CERN进行ATLAS实验的物理学家发现了第一个高能光与光直接散射的证据,光与光散射是非常罕见的过程,在此过程中两个光子相互作用、改变方向。相关研究结果已经于8月15日在《自然物理》（Nature Physics）杂志发表,结果证实了最古老的量子电动力学(quantum electrodynamics简称QED)的预言之一。更多信息请浏览原文或者相关报道。

ATLAS observes direct evidence of light-by-light scattering

August 15, 2017 by Katarina Anthony

"This is a milestone result: the first direct evidence of light interacting with itself at high energy," says Dan Tovey(University of Sheffield), ATLAS Physics Coordinator. "This phenomenon is impossible in classical theories of electromagnetism; hence this result provides a sensitive test of our understanding of QED, the quantum theory of electromagnetism."

Direct evidence for light-by-light scattering at high energy had proven elusive for decades – until the Large Hadron Collider's second run began in 2015. As the accelerator collided lead ions at unprecedented collision rates, obtaining evidence for light-by-light scattering became a real possibility. "This measurement has been of great interest to the heavy-ion and high-energy physics communities for several years, as calculations from several groups showed that we might achieve a significant signal by studying lead-ion collisions in Run 2," says Peter Steinberg (Brookhaven National Laboratory), ATLAS Heavy Ion Physics Group Convener.

Heavy-ion collisions provide a uniquely clean environment tostudy light-by-light scattering. As bunches of lead ions are accelerated, an enormous flux of surrounding photons is generated. When ions meet at the centre of the ATLAS detector, very few collide, yet their surrounding photons can interact and scatter off one another. These interactions are known as 'ultra-peripheral collisions'.

Studying more than 4 billion events taken in 2015, the ATLAS collaboration found 13 candidates for light-by-light scattering. This result has a significance of 4.4 standard deviations, allowing the ATLAS collaboration to report the first direct evidence of this phenomenon at high energy.

"Finding evidence of this rare signature required the development of a sensitive new 'trigger' for the ATLAS detector," says Steinberg. "The resulting signature—two photons in an otherwise empty detector—is almost the diametric opposite of the tremendously complicated eventstypically expected from lead nuclei collisions. The new trigger's success in selecting these events demonstrates the power and flexibility of the system, as well as the skill and expertise of the analysis and trigger groups who designed and developed it."

ATLAS physicists will continue to study light-by-light scattering during the upcoming LHC heavy-ion run, scheduled for 2018. More data will further improve the precision of theresult and may open a new window to studies of new physics. In addition, the study of ultra-peripheral collisions should play a greater role in the LHC heavy-ion programme, as collision rates further increase in Run 3 and beyond.

Explore further:Scientists find evidence for light-by-light scattering, long standing prediction of the Standard Model

More information: M. Aaboud et al. Evidence for light-by-light scattering in heavy-ion collisions with the ATLAS detector at the LHC, Nature Physics (2017). DOI: 10.1038/nphys4208

Abstract

Light-by-light scattering (γγ γγ) is a quantum-mechanical process that is forbidden in the classical theory of electrodynamics. This reaction is accessible at the Large Hadron Collider thanks to the large electromagnetic field strengths generated by ultra-relativistic colliding lead ions. Using 480 μb⁻¹ of lead–lead collision data recorded at a centre-of-mass energy per nucleon pair of 5.02 TeV by the ATLAS detector, here we report evidence for light-by-light scattering. A total of 13 candidate events were observed with an expected background of 2.6 ± 0.7 events. After background subtraction and analysis corrections, the fiducial cross-section of the process Pb + Pb (γγ) Pb^(∗) +  Pb^(∗)γγ, for photon transverse energy ET > 3 GeV, photon absolute pseudorapidity |η| < 2.4, diphoton invariant mass greater than 6 GeV, diphoton transverse momentum lower than 2 GeV and diphoton acoplanarity below 0.01, is measured to be 70 ± 24 (stat.) ± 17 (syst.) nb, which is in agreement with the standard model predictions.