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Category: particle physics – Page 220

The discovery of the Higgs boson in 2012 marked a significant milestone in particle physics. Since then, researchers at the ATLAS and CMS Collaborations have been diligently investigating its properties and probing for rare production and decay channels. Among the rare decays, the process where the Higgs boson decays into a Z boson and a photon (H → Zγ) has raised considerable attention, especially given the significant dataset collected during Run 2 of the Large Hadron Collider. Figure 1: The Zγ invariant mass distribution of events from all ATLAS and CMS analysis categories. The data (circles with error bars) in each category are weighted by ln(1 + S/B) and summed, where S and B are the observed signal and background yields in that category and in the 120–130 GeV interval, derived from the fit to data. The fitted signal-plus-background (background) terms are represented by a red solid (blue dashed) line. In the lower panel, the data and the two models are compared after subtraction of the estimated background. (Image: CERN) This is a special kind of decay – the Higgs boson does not couple directly to the Zγ pair; instead, the decay proceeds via an intermediate ‘loop’ of virtual particles. Thus, in the Standard Model, the decay probability (or branching fraction) for H → Zγ is predicted to be small – around 1.5 ×10–3, for a Higgs boson mass near 125 GeV. Theories that go beyond the Standard Model predict this branching fraction to deviate, as new particles interacting with the Higgs boson may also contribute to this loop. Exploring these variations provides valuable insights into both physics beyond the Standard Model and the nature of the Higgs boson. The ATLAS and CMS Collaborations have independently conducted extensive searches for the H → Zγ process. Both searches employ similar strategies, reconstructing the Z boson through its decays into pairs of electrons or muons. Signal events are identified as a narrow peak in the Zγ invariant mass distribution. To enhance the sensitivity, researchers exploited the most frequent Higgs-boson production modes and categorised events based on the characteristics of these production processes. They also used advanced machine-learning techniques, such as boosted decision trees, to distinguish between signal and background events. The ATLAS and CMS Collaborations have joined forces to report first evidence of the H → Zγ decay, with a significance of 3.4 standard deviations. Figure 2: Negative log-likelihood scan of the signal strength μ from the analysis of ATLAS data (blue line), CMS data (red line), and the combined result (black line). (Image: CERN) Recognising the importance of this decay channel, the ATLAS and CMS Collaborations joined forces to maximise the statistical power and sensitivity of their analyses. By combining the data sets collected by both experiments during Run 2 of the LHC (2015−2018), researchers have significantly increased the statistical precision and expanded the reach of their search. This collaborative effort allowed for a more precise and robust measurement. Figure 1 displays the observed distribution of the mass of the Zγ system in the combined data sample. Figure 2 presents the negative log-likelihood scan to identify the most likely signal strength that best describes the observed data. The signal strength (μ) is defined as the ratio of the Higgs-boson production cross-section times the H → Zγ decay branching fraction to its Standard-Model prediction. This analysis reveals evidence for the H → Zγ decay, with a significance of 3.4 standard deviations. This means that the probability that this signal is actually caused by a statistical fluctuation is smaller than 0.04%. The measured branching fraction for H → Zγ is 3.4 ± 1.1 ×10–3 and the observed signal yield is measured to be 2.2 ± 0.7 times the Standard-Model prediction. This means that the decay is seen a little more than twice as often as would be expected by the Standard Model. Although the uncertainty on the present measurement is still quite large, these findings open the door to valuable insights into the behaviour and properties of the Higgs boson. Looking ahead, by the end of LHC Run 3 the collected data is expected to triple the size of the dataset analysed here. This will allow ATLAS and CMS researchers to study this rare decay channel in even more detail, and to use this channel to probe for new physics beyond the Standard Model. About the event display: Event display of a candidate H→ Zγ event with the Z boson decaying μ+μ-. The transverse momenta of the two muon candidates, shown in red. The photon candidate is reconstructed as an unconverted photon with a transverse momentum of 32.5 GeV. Two jets are represented by light blue cones. The green boxes correspond to energy deposits in cells of the electromagnetic calorimeter, while yellow boxes correspond to energy deposits in cells of the hadronic calorimeter. Learn more Evidence for the Higgs boson decay to a Z boson and a photon at the LHC (ATLAS-CONF-2023–025) LHCP 2023 presentation by Toyoko Orimoto: Higgs boson rare production and decay at ATLAS and CMS LHCP 2023 presentation by Chiara Arcangeletti: Measurement of Higgs boson production and properties LHC experiments see first evidence for rare Higgs boson decay into two different bosons, CMS briefing, May 2023 ATLAS Collaboration: A search for the Zγ decay mode of the Higgs boson in pp collisions at 13 TeV with the ATLAS detector (Phys. Lett. B 809 (2020) 135,754, arXiv: 2005.05382, see figures) CMS Collaboration: Search for Higgs boson decays to a Z boson and a photon in proton–proton collisions at 13 TeV (Accepted for publication in J. High Energy Phys, arXiv: 2204.12945, see figures) ATLAS searches for rare Higgs boson decays into a photon and a Z boson, ATLAS Physics Briefing, April 2020 A possible new decay mode of the Higgs boson, CMS briefing, April 2022 Summary of new ATLAS results from LHCP 2023, ATLAS News, May 2023.

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The discovery of the Higgs boson at CERN’s Large Hadron Collider (LHC) in 2012 marked a significant milestone in particle physics. Since then, the ATLAS and CMS collaborations have been diligently investigating the properties of this unique particle and searching to establish the different ways in which it is produced and decays into other particles.

At the Large Hadron Collider Physics (LHCP) conference this week, ATLAS and CMS report how they teamed up to find the first evidence of the rare process in which the Higgs decays into a Z boson, the electrically neutral carrier of the weak force, and a photon, the carrier of the electromagnetic force. This Higgs boson decay could provide indirect evidence of the existence of particles beyond those predicted by the Standard Model of .

The decay of the Higgs boson into a Z boson and a photon is similar to that of a decay into two photons. In these processes, the Higgs boson does not decay directly into these pairs of particles. Instead, the decays proceed via an intermediate “loop” of “virtual” particles that pop in and out of existence and cannot be directly detected. These virtual particles could include new, as yet undiscovered particles that interact with the Higgs boson.

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Researchers have experimentally demonstrated a new quantum information storage protocol that can be used to create Greenberger-Horne-Zeilinger (GHZ) quantum states. There is a great deal of interest in these complex entangled states because of their potential use in quantum sensing and quantum error correction applications.

Chun-Ju Wu from the California Institute of Technology will present this research at the Optica Quantum 2.0 Conference and Exhibition, as a hybrid event June 18–22 in Denver, Colorado.

Quantum-based technologies store information in the form of qubits, the quantum equivalent of the binary bits used in classical computing. GHZ states take this a step further by entangling three or more qubits. This increased complexity can be used to store more information, thus boosting precision and performance in applications such as quantum sensing and networking.

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If there are energy fluctuations in a quantum vacuum, very interesting things can happen. For example, the E = mc2 relation tells us that energy and matter are interconvertible. A vacuum energy fluctuation can be converted into particles of matter. Sounds weird? Maybe, but it happens all the time. These particles are called virtual particles, living a fleeting existence before plunging back into the ever-busy quantum vacuum.

Tryon extrapolated the idea of quantum fluctuations to the Universe as a whole. He reasoned that if all that existed was a quantum vacuum, a bubble-like energy fluctuation out of this vacuum could have given rise to the Universe. Tryon proposed that the whole Universe is the result of a vacuum fluctuation, originating from what we could call quantum nothingness.

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From eyeglasses to space telescopes, lenses play crucial roles in technologies ranging from the mundane to the cutting edge. While traditional refractive lenses are a fundamental building block of optics, they are bulky and this can restrict how they are used. Metalenses are much thinner than conventional lenses and in the last two decades plenty of light has been shone on the potential of these devices, which sparkle as a promising alternative.

Metalenses are thin structures made of arrays of “meta-atoms”, which are motifs with dimensions that are smaller than the wavelength of light. It is these meta-atoms that interact with light and change its direction of propagation.

Unlike conventional refractive lenses, metalenses can be less than one micron thick, reducing the overall volume of optical systems. They can also provide ideal diffraction-limited focusing performance, while avoiding some problems associated with refractive lenses such as aberrations.

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When most people think of crystals, they picture suncatchers that act as rainbow prisms or the semi-transparent stones that some believe hold healing powers. However, to scientists and engineers, crystals are a form of materials in which their constituents—atoms, molecules, or nanoparticles—are arranged regularly in space. In other words, crystals are defined by the regular arrangement of their constituents. Common examples are diamonds, table salt, or sugar cubes.

However, in research just published in Soft Matter, a team led by Rensselaer Polytechnic Institute’s Sangwoo Lee, associate professor in the Department of Chemical and Biological Engineering, discovered that crystal structures are not necessarily always regularly arranged. The discovery advances the field of materials science and has unrealized implications for the materials used for semiconductors, solar panels, and electric vehicle technologies.

One of the most common and important classes of crystal structures is the close-packed structures of regular spheres constructed by stacking layers of spheres in a honeycomb arrangement. There are many ways to stack the layers to construct close-packed structures, and how nature selects specific stacking is an important question in materials and physics research. In the close-packing construction, there is a very unusual structure with irregularly spaced constituents known as the random stacking of two-dimensional hexagonal layers (RHCP). This structure was first observed from cobalt metal in 1942, but it has been regarded as a transitional and energetically unpreferred state.

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Scientists at EPFL have found a new way to create a crystalline structure called a “density wave” in an atomic gas. The findings can help us better understand the behavior of quantum matter, one of the most complex problems in physics. The research was published May 24 in Nature.

“Cold atomic gases were well known in the past for the ability to ‘program’ the interactions between atoms,” says Professor Jean-Philippe Brantut at EPFL. “Our experiment doubles this ability.” Working with the group of Professor Helmut Ritsch at the University of Innsbruck, they have made a breakthrough that can impact not only quantum research but quantum-based technologies in the future.

Scientists have long been interested in understanding how materials self-organize into complex structures, such as crystals. In the often-arcane world of quantum physics, this sort of self-organization of particles is seen in “,” where particles arrange themselves into a regular, repeating pattern or order; like a group of people with different colored shirts on standing in a line but in a pattern where no two people with the same color shirt stand next to each other.

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A curved “laser wakefield accelerator” could boost the acceleration potential of a multistage version of this device.

Laser wakefield accelerators (LWFAs) use laser-generated plasmas to accelerate electrons to high energies. The devices are significantly smaller than radio-frequency-based particle accelerators—centimeters versus hundreds of meters—making them less expensive, more efficient alternatives. But researchers still need to demonstrate that LWFAs can achieve particle energies that match those of their conventional counterparts. Now Xinzhe Zhu from Shanghai Jiao Tong University and colleagues have brought that goal a step closer, demonstrating a method for linking multiple LWFAs in a way that would boost their acceleration potential [1].

In an LWFA, charged particles reach relativistic speeds by surfing a wave of plasma created by a powerful laser. The particle energy achievable with a single LWFA is limited to a few GeV for two reasons: the particle bunch and the plasma wave quickly fall out of sync, and the laser energy dissipates with distance. Routing particles through multiple connected LWFAs would overcome these problems. But current techniques for combining LWFAs require refocusing the beam at each connection, lowering the efficiency of the process.

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It may be that the famous Higgs boson, co-responsible for the existence of masses of elementary particles, also interacts with the world of the new physics that has been sought for decades. If this were indeed to be the case, the Higgs should decay in a characteristic way, involving exotic particles. At the Institute of Nuclear Physics of the Polish Academy of Sciences in Cracow, it has been shown that if such decays do indeed occur, they will be observable in successors to the LHC currently being designed.

When talking about the ‘hidden valley’, our first thoughts are of dragons rather than sound science. However, in high-energy physics, this picturesque name is given to certain models that extend the set of currently known elementary particles. In these so-called Hidden Valley models, the particles of our world as described by the Standard Model belong to the low-energy group, while exotic particles are hidden in the high-energy region. Theoretical considerations suggest then the exotic decay of the famous Higgs boson, something that has not been observed at the LHC accelerator despite many years of searching. However, scientists at the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Cracow argue that Higgs decays into exotic particles should already be perfectly observable in accelerators that are successors to the Large Hadron Collider – if the Hidden Valley models turn out to be consistent with reality.

“In Hidden Valley models we have two groups of particles separated by an energy barrier. The theory is that there could then be exotic massive particles that could cross this barrier under specific circumstances. The particles like Higgs boson or hypothetic Z’ boson would act as communicators between the particles of both worlds. The Higgs boson, one of the most massive particle of the Standard Model, is a very good candidate for such a communicator,” explains Prof. Marcin Kucharczyk (IFJ PAN), lead author of an article in the Journal of High Energy Physics, which presents the latest analyses and simulations concerning the possibility of detecting Higgs boson decays in the future lepton accelerators.

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