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Department of Physics

The Cavendish Laboratory
 

Tue 15 Jun 16:00: Searches for New Physics with Top Quarks at ATLAS: A review from LHCP and beyond

Talks - Tue, 08/06/2021 - 15:07
Searches for New Physics with Top Quarks at ATLAS: A review from LHCP and beyond

As the most massive discovered fundamental particle, the top quark has a unique potential to couple to physics new physics at or beyond the TeV scale. In the current ‘golden age’ for top physics, with the LHC often described as a top quark factory, ATLAS has a rich program of research that utilises the top in a variety of ways to hunt for Beyond the Standard Model phenomena. This talk will focus on some of the most recent exploratory ATLAS results presented at LHCP , covering both direct searches for specific models, and model-independent EFT fits on precision measurements. A particular focus is given to rare associated top quark production modes, which represent the furthest reaches of the ATLAS top program to date.

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Tue 15 Jun 16:00: Overview of ATLAS top physicsSearches for New Physics with Top Quarks at ATLAS: A review from LHCP and beyond

Talks - Tue, 08/06/2021 - 11:32
Overview of ATLAS top physicsSearches for New Physics with Top Quarks at ATLAS: A review from LHCP and beyond

As the most massive discovered fundamental particle, the top quark has a unique potential to couple to physics new physics at or beyond the TeV scale. In the current ‘golden age’ for top physics, with the LHC often described as a top quark factory, ATLAS has a rich program of research that utilises the top in a variety of ways to hunt for Beyond the Standard Model phenomena. This talk will focus on some of the most recent exploratory ATLAS results presented at LHCP , covering both direct searches for specific models, and model-independent EFT fits on precision measurements. A particular focus is given to rare associated top quark production modes, which represent the furthest reaches of the ATLAS top program to date.

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Atom swapping could lead to ultra-bright, flexible next generation LEDs

Physics News - Mon, 07/06/2021 - 16:25

The researchers, led by the University of Cambridge and the Technical University of Munich, found that by swapping one out of every one thousand atoms of one material for another, they were able to triple the luminescence of a new material class of light emitters known as halide perovskites.  

This ‘atom swapping’, or doping, causes the charge carriers to get stuck in a specific part of the material’s crystal structure, where they recombine and emit light. The results, reported in the Journal of the American Chemical Society, could be useful for low-cost printable and flexible LED lighting, displays for smartphones or cheap lasers.

Many everyday applications now use light-emitting devices (LEDs), such as domestic and commercial lighting, TV screens, smartphones and laptops. The main advantage of LEDs is they consume far less energy than older technologies.

Ultimately, also the entirety of our worldwide communication via the internet is driven by optical signals from very bright light sources that within optical fibres carry information at the speed of light across the globe.

The team studied a new class of semiconductors called halide perovskites in the form of nanocrystals which measure only about a ten-thousandth of the thickness of a human hair. These ‘quantum dots’ are highly luminescent materials: the first high-brilliance QLED TVs incorporating quantum dots recently came onto the market.

The Cambridge researchers, working with Daniel Congreve’s group at Harvard, who are experts in the fabrication of quantum dots, have now greatly improved the light emission from these nanocrystals. They substituted one out of every one thousand atoms with another – swapping lead for manganese ions – and found the luminescence of the quantum dots tripled.

A detailed investigation using laser spectroscopy revealed the origin of this observation. “We found that the charges collect together in the regions of the crystals that we doped,” said Sascha Feldmann from Cambridge’s Cavendish Laboratory, the study’s first author. “Once localised, those energetic charges can meet each other and recombine to emit light in a very efficient manner.”

“We hope this fascinating discovery: that even smallest changes to the chemical composition can greatly enhance the material properties, will pave the way to cheap and ultrabright LED displays and lasers in the near future,” said senior author Felix Deschler, who is jointly affiliated at the Cavendish and the Walter Schottky Institute at the Technical University of Munich.

In the future, the researchers hope to identify even more efficient dopants which will help make these advanced light technologies accessible to every part of the world.

 

Reference:
Sascha Feldmann et al. ‘Charge carrier localization in doped perovskite nanocrystals enhances radiative recombination.’, Journal of the American Chemical Society (2021). DOI: 10.1021/jacs.1c01567

An international group of researchers has developed a new technique that could be used to make more efficient low-cost light-emitting materials that are flexible and can be printed using ink-jet techniques.

Ella Maru StudioArtist’s impression of glowing halide perovskite nanocrystals


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Fri 11 Jun 16:00: Leading hadronic contribution to the muon magnetic moment from lattice quantum chromodynamics

Talks - Mon, 07/06/2021 - 08:38
Leading hadronic contribution to the muon magnetic moment from lattice quantum chromodynamics

The seminar will take place via Zoom here.

Abstract: Twenty years ago in an experiment at Brookhaven National Laboratory, physicists measured the muon’s anomalous magnetic moment, $a_\mu=(g_\mu-2)/2$, with a remarkable precision of 0.54 parts per million. Since then, the standard model prediction for $a_\mu$ has exhibited a discrepancy with experiment of over 3 standard deviations, raising the tantalizing possibility of physical particles or forces as yet undiscovered. On April 7 a new experiment at Fermilab presented its first results, brilliantly confirming Brookhaven’s measurement and bringing the discrepancy with the standard model to a near discovery level of 4.2 sigma. To fully leverage this and future measurements, and possibly claim the presence of new fundamental physics, it is imperative to check the standard model prediction with independent methods, and to reduce its uncertainties. After an introduction and a discussion of the current experimental and theoretical status of $a_\mu$, I will present a precise lattice QCD calculation, by the BMW collaboration, of the contribution to this quantity that most limits the precision of the standard model prediction. The result of this calculation significantly reduces the gap between the standard model and experiment, and suggests that new physics may not be needed to explain the current, experimental, world-average value of $a_\mu$.

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Thu 24 Jun 16:00: A puzzle in $B\to D^{+*}{\pi,K}$ decays: is it New Physics?

Talks - Fri, 04/06/2021 - 17:42
A puzzle in $B\to D^{+*}{\pi,K}$ decays: is it New Physics?

Abstract: The determination of the branching fractions of non-leptonic B decays potentially suffers from large uncertainties. However, among the possible modes, the B → (D+\star) K and B_s → (D_s+\star) pi decays are the cleanest, and recent updates on the inputs needed to predict their branching fraction unveiled a ~ 4sigma discrepancy with respect to the current experimental measurements. A possible explanation of this puzzle is in terms of New Physics particles. In this presentation, I will review the theoretical progress that lead to this puzzle. I will then move to present possible simplified models that can accommodate this puzzle and the constraining power of high-pT data for these scenarios.

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Fri 18 Jun 16:00: TBA

Talks - Fri, 04/06/2021 - 17:41
TBA

The seminar will take place via Zoom here.

Abstract:

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Thu 17 Jun 14:00: Detecting fractional chern insulators of few-bosons

Talks - Fri, 04/06/2021 - 17:10
Detecting fractional chern insulators of few-bosons

Realizing strongly correlated topological phases of ultracold gases is a central goal for ongoing experiments. While fractional quantum Hall states could soon be implemented in small atomic ensembles, detecting their signatures in few-particle settings remains a fundamental challenge. In this work, we numerically analyze the center-of-mass Hall drift of a small ensemble of hardcore bosons, initially prepared in the ground state of the Harper-Hofstadter-Hubbard model in a box potential. By monitoring the Hall drift upon release, for a wide range of magnetic flux values, we identify an emergent Hall plateau compatible with a fractional Chern insulator state: The extracted Hall conductivity approaches a fractional value determined by the many-body Chern number, while the width of the plateau agrees with the spectral and topological properties of the prepared ground state. Besides, a direct application of Streda’s formula indicates that such Hall plateaus can also be directly obtained from static density-profile measurements. Our calculations suggest that fractional Chern insulators can be detected in cold-atom experiments, using available detection methods.

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Thu 10 Jun 14:00: Symmetric Jordan-Wigner transformation on the square lattice

Talks - Tue, 01/06/2021 - 10:06
Symmetric Jordan-Wigner transformation on the square lattice

Interpreted in a restricted sense, the Jordan-Wigner transformation provides a manifestly local representation of a lattice fermionic system using bosonic degrees of freedom. While locality becomes an issue in higher than one spatial dimension in the original formulation, recently it has been shown that the problem can be resolved by invoking a background Z2 gauge theory. Building on these recent developments, we will discuss a way to perform the Jordan-Wigner transformation while keeping not only locality but also the symmetries manifest.

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Tue 01 Jun 16:00: Lepton Flavour Universality - measuring BR(W->τν)/BR(W->μν) at the LHC

Talks - Mon, 31/05/2021 - 15:14
Lepton Flavour Universality - measuring BR(W->τν)/BR(W->μν) at the LHC

In the Standard Model on of the fundamental principles is that the couplings of the W-boson to the different lepton flavours (e, mu, tau) should be identical. Therefore, outside of small phase space effects, the branching ratio of W->lν should be identical for all flavours of lepton. The most precise previous measurements of the branching ratios of the W boson to the different lepton flavours were from the Large Electron-Positron Collider (LEP) at CERN and showed some tension with the Standard Model. This seminar will describe an ATLAS analysis at the LHC measuring the ratio BR(W->τν)/BR(W->μν) with higher precision than the previous LEP measurements to test this tension with the Standard Model expectation. Top—anti-Top events are used to provide a large sample of W-bosons with which to perform the measurement. One top is then used to trigger and tag the events and the other is used to test the ratio in question.

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Wed 16 Jun 11:30: Systematic study of magnetotransport responses with Berry-Boltzmann equations and Wannier functions

Talks - Fri, 28/05/2021 - 15:49
Systematic study of magnetotransport responses with Berry-Boltzmann equations and Wannier functions

There has recently been a huge renewed interest in the literature to the study of currents generated in solids by external electrical and magnetic fields, especially in relation with topology, Berry curvature and Weyl points. The responses of interest include nonlinear anomalous Hall effect, crystal Hall effect, planar Hall effect, unidirectional magnetoresistance and electrical magnetochiral anisotropy among others. In this talk I would like to show how all these responses are classified according to their properties with respect to the inversion and time-reversal symmetry, and how they can be systematically described by the Boltzmann equations modified by Berry curvature and intrinsic magnetic moment of Bloch states – the so-called “Berry-Boltzmann” equations. In addition the different terms contributing to the conductivity tensors are classified due to there microscopic origin as being classical or due to Berry curvature and/or intrinsic magnetic moment. I will also discuss what we should call “Hall” and “Ohmic” conductivities in the nonlinear responses, and how they may be extracted from a particular conductivity tensor. If time permits, I will also address some aspects of ab initio evaluation of such responses within Wannier functions formalism, and how we implement it in the WannierBerri code.

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Fri 28 May 16:00: Towards NNLO QCD+NLO EW PDF fits with NNPDF

Talks - Fri, 28/05/2021 - 15:48
Towards NNLO QCD+NLO EW PDF fits with NNPDF

The seminar will take place via Zoom here.

Abstract: I will present an effort within the NNPDF collaboration to perform a PDF fit including, for the first time, a fully consistent treatment of higher-order EW corrections. As PDFs are determined more or more precisely, the effects of previously neglected corrections are beginning to be important. This implies the inclusion of EW corrections for every theoretical prediction going into the PDF fit, and, on the other hand, a different treatment of the measured observables. Finally, a few technical tools had to be developed, which I am going to present as well.

The slides are available here

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Fri 04 Jun 16:00: A model of muon anomalies

Talks - Thu, 27/05/2021 - 16:05
A model of muon anomalies

The seminar will take place via Zoom here.

Recent R(K) update from the LHCb experiment at CERN reinforced the tension of B-meson decays into muons. The Muon g-2 experiment at Fermilab strengthened the tension in the muon anomalous magnetic moment. Can muon anomalies be coherently addressed in models beyond the SM, and if so, where else should we look for confirmation? I will discuss extensions of the SM based on 2103.13991 and some work in progress.

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Mind Over Chatter: What is the future?

Physics News - Thu, 27/05/2021 - 14:22
Season 2, episode 1

This second series of Mind Over Chatter is all about the future - and in this first episode we’re going to be considering what the future even is… Have you ever wondered how time works? It turns out, the answer is a lot more complicated than we thought.

Join our wondering and wonderful conversation with philosopher of science Dr Matt Farr, whose work focuses particularly on what it means for time to have a direction, professor of psychology Nicky Clayton, who looks at the evolution and development of intelligence in non-verbal animals and pre-verbal children, and professor of linguistics and philosophy, Kasia Jaszczolt whose research interests combine semantics, pragmatics, and the metaphysics of time 

We’ll be talking about everything from physics to linguistics… and from broken eggs to Einstein’s theory of relativity. 

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Key points

[04:28] - Does time actually go from past to present to future? And does time really ‘flow’?

[09:53] - How do B-theorists deal with entropy? Can you un-break an egg?

[14:12] - Recap of the first portion of the episode, reviewing A-theory, B-theory and C-theory of time

[18:58] - How the mind understands the subjective concept of time

[27:11] - The Sapir-Whorf Hypothesis and how the way you talk about language affects the way you perceive and think about things

[30:21] - Recap of the second portion of the episode 

[34:02] - How do the mental and linguistic concepts around time fit with philosophical concepts and physics of time?

[45:46] - Is there a conflict between the psychological and linguistic models of time and the way physics handles time?

[48:20] - Recap of the last portion of the episode

Mind Over Chatter: The Cambridge University Podcast

Wed 09 Jun 16:00: First principles simulations of electrolyte materials with a view toward all solid state battery technology

Talks - Wed, 26/05/2021 - 16:25
First principles simulations of electrolyte materials with a view toward all solid state battery technology

The development of an international consensus on the need to pay attention to conserving our fragile planet has inspired many computational research efforts in the condensed matter community. While several of these effort focus on predicting new materials, another approach focuses on detailed analyses of the properties of known materials. This latter approach, while often more of academic interest not necessarily leading to promising technological breakthroughs, is important for determining and refining the relationships of simulation techniques to modeling properties of real material. This talk presents the example study of the alkali-metal hexathiohypodiphosphate materials Li4P2S6 and Na4P2S6 and their alloys which are of interest to the effort of developing all solid state batteries. While Li4P2S6 has been identified as an unwanted decomposition product in the preparation of lithium thiophosphate electrolytes with very poor ionic conductivity, Na4P2S6 appears to be a competitive electrolyte for sodium ion batteries. With the help of Yan Li (WFU) and Zach Hood (ANL) as well as several others, the study of these compounds over several years has finally lead to a plausible understanding of their structures and properties. Ref. Phys. Rev. Materials 4, 045406 (2020).

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Wed 02 Jun 11:30: Non-Abelian braiding of phonons in layered silicates

Talks - Wed, 26/05/2021 - 10:11
Non-Abelian braiding of phonons in layered silicates

One of the most promising ideas for the realization of topological qubits relies on non-Abelian anyons. However, the extremely subtle conditions to detect the Majorana bound states hamper technologically relevant applications. Here we propose that phonons, a bosonic excitation, can realize momentum space braiding of band nodes that carry non-Abelian charges [1]. Different from electronic systems, phonons do not have the constraints placed by the Fermi-Dirac distribution that have so far prevented the experimental observation of multi-gap topologies in real materials. We find a promising material family, layered silicates that are ubiquitous in soils and minerals throughout the world, to study the non-Abelian braiding of phonons. The associated braiding process can be controlled by means of an electric field and epitaxial strain, and involves, for the first time, more than three bands. Finally, we propose that these conversion processes can be tracked by following the evolution of the Raman spectrum, providing a clear signature for the experimental verification of multi-gap topologies.

Reference: [1] B Peng, A Bouhon, B Monserrat, RJ Slager. Non-Abelian braiding of phonons in layered silicates. arXiv:2105.08733

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Tue 25 May 16:00: Overview of quark gluon tagging

Talks - Mon, 24/05/2021 - 12:01
Overview of quark gluon tagging

The ability to distinguish quark-initiated jets from those seeded by gluons would greatly enhance the physics potential of the LHC , including searches for BSM physics and measurements of the Standard Model. This long-standing puzzle has had many techniques and methods applied to it and in this seminar the various approaches over the years at ATLAS and CMS are summarised.

Previous efforts used variables that aimed to exploit the observable differences between quark and gluon jets; including, but by no means limited to, the charged track multiplicity. Quantities with the most separation power are often combined to form two dimensional likelihoods which are more performant. However, in the modern age of machine learning techniques quark gluon-tagging is an ideal use case for these sophisticated methods.

How will the power of these discriminators increase into Run 3 of the LHC and beyond to the High Luminosity LHC ? Is the separation of these highly entangled theoretical abstractions even a reasonably achievable goal experimentally?

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Fri 28 May 16:00: Towards NNLO QCD+NLO EW PDF fits with NNPDF

Talks - Mon, 24/05/2021 - 09:58
Towards NNLO QCD+NLO EW PDF fits with NNPDF

The seminar will take place via Zoom here.

Abstract: I will present an effort within the NNPDF collaboration to perform a PDF fit including, for the first time, a fully consistent treatment of higher-order EW corrections. As PDFs are determined more or more precisely, the effects of previously neglected corrections are beginning to be important. This implies the inclusion of EW corrections for every theoretical prediction going into the PDF fit, and, on the other hand, a different treatment of the measured observables. Finally, a few technical tools had to be developed, which I am going to present as well.

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Fri 21 May 16:00: POSTPONED: A puzzle in $B\to D^{+*}{\pi,K}$ decays: is it New Physics?

Talks - Wed, 19/05/2021 - 14:09
POSTPONED: A puzzle in $B\to D^{+*}{\pi,K}$ decays: is it New Physics?

This seminar will be postponed to the end of term

Abstract: The determination of the branching fractions of non-leptonic B decays potentially suffers from large uncertainties. However, among the possible modes, the B → (D+\star) K and B_s → (D_s+\star) pi decays are the cleanest, and recent updates on the inputs needed to predict their branching fraction unveiled a ~ 4sigma discrepancy with respect to the current experimental measurements. A possible explanation of this puzzle is in terms of New Physics particles. In this presentation, I will review the theoretical progress that lead to this puzzle. I will then move to present possible simplified models that can accommodate this puzzle and the constraining power of high-pT data for these scenarios.

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Thu 20 May 14:00: Low-temperature quantum bounds in simple models

Talks - Sat, 15/05/2021 - 09:50
Low-temperature quantum bounds in simple models

In the past few years there has been considerable activity around a set of quantum bounds on transport coefficients (viscosity, conductivity) and on chaos (Lyapunov exponents), relevant at low temperatures. The interest comes from the fact that AdS/CFT Black-Hole models seem to saturate all of them. To fix ideas, a simple case is that of systems whose lowest energy is a degenerate manifold, rather than a point. Examples are quantum hard-sphere liquids and quantum spin liquids. In this context the bounds are approached and are consequences of the uncertainty principle, and one understands the mechanisms whereby quantum mechanics enforces them. For a system to saturate the bound, it appears as a necessary condition that at each temperature there are some degrees of freedom that are still classical, and some are on the verge of being affected by quantum effects.

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