Natural Philosophy versus Philosophy of Naturalness

Goran Senjanovic
I reflect on some of the basic aspects of present day Beyond the Standard Model particle physics, focusing mostly on the issues of naturalness, in particular on the so-called hierarchy problem. To all of us, physics as natural science emerged with Galileo and Newton, and led to centuries of unparalleled success in explaining and often predicting new phenomena of nature. I argue here that the long standing obsession with the hierarchy problem as a guiding principle for the future of our field has had the tragic consequence of deviating high energy physics from its origins as natural philosophy, and turning it into a philosophy of naturalness.
Read more at https://arxiv.org/pdf/2001.10988.pdf

50 years of the GIM mechanism

Hong-Jian He, John Ellis, John Iliopoulos, Sheldon Lee Glashow, Verónica Riquer and Luciano Maiani at a celebration of 50 years of the GIM mechanism in Shanghai. Credit: J Liu

In 1969 many weak amplitudes could be accurately calculated with a model of just three quarks, and Fermi’s constant and the Cabibbo angle to couple them. One exception was the remarkable suppression of strangeness-changing neutral currents. John Iliopoulos, Sheldon Lee Glashow and Luciano Maiani boldly solved the mystery using loop diagrams featuring the recently hypothesised charm quark, making its existence a solid prediction in the process. To celebrate the fiftieth anniversary of their insight, the trio were guests of honour at an international symposium at the T. D. Lee Institute at Shanghai Jiao Tong University on 29 October, 2019.

The UV cutoff needed in the three-quark theory became an estimate of the mass of the fourth quark

The Glashow-Iliopoulos-Maiani (GIM) mechanism was conceived in 1969, submitted to Physical Review D on 5 March 1970, and published on 1 October of that year, after several developments had defined a conceptual framework for electroweak unification. These included Yang-Mills theory, the universal V−A weak interaction, Schwinger’s suggestion of electroweak unification, Glashow’s definition of the electroweak group SU(2)L×U(1)Y, Cabibbo’s theory of semileptonic hadron decays and the formulation of the leptonic electroweak gauge theory by Weinberg and Salam, with spontaneous symmetry breaking induced by the vacuum expectation value of new scalar fields. The GIM mechanism then called for a fourth quark, charm, in addition to the three introduced by Gell-Mann, such that the first two blocks of the electroweak theory are made each by one lepton and one quark doublet, [(νe, e), (u, d)] and [(νµ, µ), (c, s)]. Quarks u and c are coupled by the weak interaction to two superpositions of the quarks d and s: u ↔ dC , with dC the Cabibbo combination dC = cos θC d + sin θC s, and c ↔ sC , with sC the orthogonal combination. In subsequent years, a third generation, [(ντ, τ ), (t, b)] was predicted to describe CP violation. No further generations have been observed yet.

Problem solved

The GIM mechanism was the solution to a problem arising in the simplest weak interaction theory with one charged vector boson coupled to the Cabibbo currents. As pointed out in 1968, strangeness-changing neutral-current processes, such as KL → µ+µ and K0 – K0 mixing, are generated at one loop with amplitudes of order G sinθC cosθC (GΛ2), where G is the Fermi constant, Λ is an ultraviolet cutoff, and GΛ2 (dimensionless) is the first term in a perturbative expansion which could be continued to take higher order diagrams into account. To comply with the strict limits existing at the time, one had to require a surprisingly small value of the cutoff, Λ, of 2 − 3 GeV, to be compared with the naturally expected value: Λ = G-1/2 ~ 300 GeV. This problem was taken seriously by the GIM authors, who wrote that “it appears necessary to depart from the original phenomenological model of weak interactions”.

One-loop quark diagrams for K0 – K0 mixing in the light of the GIM mechanism. The charm-quark amplitudes have the same magnitude but opposite sign as for up-quark lines, leading to a perfect cancellation, cos θ sin θ + (- sin θ) cos θ = 0, in the case where mc = mu and suggesting an explanation for the suppression of processes with strangeness-changing neutral currents.

To sidestep this problem, Glashow, Iliopoulos and Maiani brought in the fourth “charm” quark, already introduced by Bjorken, Glashow and others, with its typical coupling to the quark combination left alone in the Cabibbo theory: c ↔ sC = − sinθC d + cosθC s. Amplitudes for s → d with u or c on the same fermion line would cancel exactly for m= mu, suggesting a more natural means to suppress strangeness-changing neutral-current processes to measured levels. For m>> mu, a residual neutral-current effect would remain, which, by inspection, and for dimensional reasons, is of order G sinθC cos θC (GMc2). This was a real surprise: the “small” UV cutoff needed in the simple three-quark theory became an estimate of the mass of the fourth quark, which was indeed sufficiently large to have escaped detection in the unsuccessful searches for charmed mesons that had been conducted in 1960s. With the two quark doublets included, a detailed study of strangeness changing neutral current processes gave mc ∼ 1.5 GeV, a value consistent with more recent data on the masses of charmed mesons and baryons. Another aspect of the GIM cancellation is that the weak charged currents make an SU(2) algebra together with a neutral component that has no strangeness changing terms. Thus, there is no difficulty to include the two quark doublets in the unified electroweak group SU(2)L×U(1)Y of Glashow, Weinberg and Salam. The 1970 GIM paper noted that “in contradistinction to the conventional (three-quark) model, the couplings of the neutral intermediary – now hypercharge conserving – cause no embarrassment.”

The GIM mechanism has become a cornerstone of the Standard Model and it gives a precise description of the observed flavour changing neutral current processes for s and b quarks. For this reason, flavour-changing neutral currents are still an important benchmark and give strong constraints on theories that go beyond the Standard Model in the TeV region.

 

FASER’s new detector expected to catch first collider neutrino

The first-of-its-kind detector could initiate a new era in neutrino physics at particle colliders

Illustration of the FASER experiment. The new FASERν detector, which is just 25 cm wide, 25 cm tall and 1.35 m long, will be located at the front of FASER’s main detector in a narrow trench (yellow block in the bottom right of the image). (Image: FASER/CERN)

No neutrino produced at a particle collider has ever been detected, even though colliders create them in huge numbers. This could now change with the approval of a new detector for the FASER experiment at CERN. The small and inexpensive detector, called FASERν, will be placed at the front of the FASER experiment’s main detector, and could launch a new era in neutrino physics at particle colliders.

Ever since they were first observed at a nuclear reactor in 1956, neutrinos have been detected from many sources, such as the sun, cosmic-ray interactions in the atmosphere, and the Earth, yet never at a particle collider. That’s unfortunate, because most collider neutrinos are produced at very high energies, at which neutrino interactions have not been well studied. Neutrinos produced at colliders could therefore shed new light on neutrinos, which remain the most enigmatic of the fundamental particles that make up matter.

The main reasons why collider neutrinos haven’t been detected are that, firstly, neutrinos interact very weakly with other matter and, secondly, collider detectors miss them. The highest-energy collider neutrinos, which are more likely to interact with the detector material, are mostly produced along the beamline – the line travelled by particle beams in a collider. However, typical collider detectors have holes along the beamline to let the beams through, so they can’t detect these neutrinos.

Enter FASER, which was approved earlier this year to search for light and weakly interacting particles such as dark photons – hypothetical particles that could mediate an unknown force that would link visible matter with dark matter. FASER, supported by the Heising-Simons and Simons Foundations, will be located along the beamline of the Large Hadron Collider (LHC), about 480 metres downstream of the ATLAS experiment, so it will be ideally positioned to detect neutrinos. However, the detection can’t be done with the experiment’s main detector.

“Since neutrinos interact very weakly with matter, you need a target with a lot of material in it to successfully detect them. The main FASER detector doesn’t have such a target, and is therefore unable to detect neutrinos, despite the huge number that will traverse the detector from the LHC collisions,” explains Jamie Boyd, co-spokesperson for the FASER experiment. “This is where FASERν comes in. It is made up of emulsion films and tungsten plates, and acts both as the target and the detector to see the neutrino interactions.”

FASERν is only 25 cm wide, 25 cm tall and 1.35 m long, but weighs 1.2 tonnes. Current neutrino detectors are generally much bigger, for example Super-Kamiokande, an underground neutrino detector in Japan, weighs 50 000 tonnes, and the IceCube detector in the South Pole has a volume of a cubic kilometre.

After studying FASER’s ability to detect neutrinos and doing preliminary studies using pilot detectors in 2018, the FASER collaboration estimated that FASERν could detect more than 20 000 neutrinos. These neutrinos would have a mean energy of between 600 GeV and 1 TeV, depending on the type of neutrino produced. Indeed there are three types of neutrinos – electron neutrino, muon neutrino and tau neutrino – and the collaboration expects to detect 1300 electron neutrinos, 20 000 muon neutrinos and 20 tau neutrinos.

“These neutrinos will have the highest energies yet of man-made neutrinos, and their detection and study at the LHC will be a milestone in particle physics, allowing researchers to make highly complementary measurements in neutrino physics,” says Boyd. “What’s more, FASERν may also pave the way for neutrino programmes at future colliders, and the results of these programmes could feed into discussions of proposals for much larger neutrino detectors.”

The FASERν detector will be installed before the next LHC run, which will start in 2021, and it will collect data throughout this run.

https://home.cern/news/news/physics/fasers-new-detector-expected-catch-first-collider-neutrino

FCC-ee: Your Questions Answered

This document answers in simple terms many FAQs about FCC-ee, including comparisons with other colliders. It complements the FCC-ee CDR and the FCC Physics CDR by addressing many questions from non-experts and clarifying issues raised during the European Strategy symposium in Granada, with a view to informing discussions in the period between now and the final endorsement by the CERN Council in 2020 of the European Strategy Group recommendations. This document will be regularly updated as more questions appear or new information becomes available.

Baseline FCC tunnel layout with a perimeter of 97.5 km, and ptimized placement in the Geneva basin, showing the main topographical and geological features.

Read more at https://arxiv.org/pdf/1906.02693.pdf

On Future High-Energy Colliders

Gian Francesco Giudice
While the LHC physics programme is still in full swing, the preparations for the European Strategy for Particle Physics and the recent release of the FCC Conceptual Design Report bring attention to the future of high-energy physics. How do results from the LHC impact the future of particle physics? Why are new high-energy colliders needed?
Read more at https://arxiv.org/pdf/1902.07964.pdf

After primordial inflation

D. V. Nanopoulos, K. A. Olive, M. Srednicki
We consider the history of the early universe in the locally supersymmetric model we have previously discussed. We pay particular attention to the requirement of converting the quanta of the field which drives primordial inflation (inflatons) to ordinary particles which can produce the cosmological baryon asymmetry without producing too many gravitinos. An inflaton mass of about 1013 GeV (a natural value in our model) produces a completely acceptable scenario.
Read more at https://lib-extopc.kek.jp/preprints/PDF/1983/8305/8305219.pdf

Black Hole as Extreme Particle Accelerator

Life of the jet set. This simulation follows along in a “co-moving” reference frame with a fixed set of particles as they are blasted out of an active galactic nucleus (AGN). The magnetic field lines they experience change as they move from a smoother region (left) to a region with a kink instability (right).  [Credit: E. P. Alves et al., Phys. Rev. Lett. (2018)]

Efficient Nonthermal Particle Acceleration by the Kink Instability in Relativistic Jets

E. Paulo Alves, Jonathan Zrake, Frederico Fiuza
Relativistic magnetized jets from active galaxies are among the most powerful cosmic accelerators, but their particle acceleration mechanisms remain a mystery. We present a new acceleration mechanism associated with the development of the helical kink instability in relativistic jets, which leads to the efficient conversion of the jet’s magnetic energy into nonthermal particles. Large-scale three-dimensional ab initio simulations reveal that the formation of highly tangled magnetic fields and a large-scale inductive electric field throughout the kink-unstable region promotes rapid energization of the particles. The energy distribution of the accelerated particles develops a well-defined power-law tail extending to the radiation-reaction limited energy in the case of leptons, and to the confinement energy of the jet in the case of ions. When applied to the conditions of well-studied bright knots in jets from active galaxies, this mechanism can account for the spectrum of synchrotron and inverse Compton radiating particles, and offers a viable means of accelerating ultra-high-energy cosmic rays to 1020 eV.

Read more at https://physics.aps.org/articles/v11/130 and https://arxiv.org/pdf/1810.05154.pdf