## Moriond 2022 : Return of the Excesses ?!

Recontres de Moriond is probably the biggest ski-vacation  conference of the year in particle physics, and is one of the places big particle physics experiments often unveil their new results. For the last few years the buzz in particle physics has been surrounding ‘indirect’ probes of new physics, specifically the latest measurement of the muons anomalous magnetic moment (g-2) and hints from LHCb about lepton flavor universality violation. If either of these anomalies were confirmed this would of course be huge, definitive laboratory evidence for physics beyond the standard model, but they would not answer the question of what exactly that new physics was. As evidenced by the 500+ papers written in the last year offering explanations of the g-2 anomaly, there are a lot of different potential explanations.

A definitive answer would come in the form of a ‘direct’ observation of whatever particle is causing the anomaly, which traditionally means producing and observing said particle in a collider. But so far the largest experiments performing these direct searches, ATLAS and CMS, have not shown any hints of new particles. But this Moriond, as the LHC experiments are getting ready for the start of a new data taking run later this year, both collaborations unveiled ‘excesses’ in their Run-2 data. These excesses, extra events above a background prediction that resemble the signature of a new particle, don’t have enough statistical significance to claim discoveries yet, and may disappear as more data is collected, as many an excess has done before. But they are intriguing and some have connections to anomalies seen in other experiments.

So while there have been many great talks at Moriond (covering cosmology, astro-particle searches for dark matter, neutrino physics, and more flavor physics measurements and more) and the conference is still ongoing, its worth reviewing these new excesses in particular and what they might mean.

## Excess 1: ATLAS Heavy Stable Charged Particles

Talk (paper forthcoming): https://agenda.infn.it/event/28365/contributions/161449/attachments/89009/119418/LeThuile-dEdx.pdf

Most searches for new particles at the LHC assume that said new particles decay very quickly once they are produced and their signatures can then be pieced together by measuring all the particles they decay to. However in the last few years there has been increasing interest in searching for particles that don’t decay quickly and therefore leave striking signatures in the detectors that can be distinguished from regular Standard Model particles. This particular ATLAS search searches for particles that are long-lived, heavy and charged. Due to their heavy masses (and/or large charges) particles such as these will produce greater ionization signals as they pass through the detector than standard model particles would. This ATLAS analysis selects tracks with high momentum, and unusually high ionization signals. They find an excess of events with high mass and high ionization, with a significance of 3.3-sigma.

If their background has been estimated properly, this seems to be quite clear signature and it might be time to get excited. ATLAS has checked that these events are not due to any known instrumental defect, but they do offer one caveat. For a heavy particle like this (with a mass of ~TeV) one would expect for it to be moving noticeably slower than the speed of light. But when ATLAS compares the ‘time of flight’ of the particle, how long it takes to reach their detectors, its velocity appears indistinguishable from the speed of light. One would expect background Standard Model particles to travel close to the speed of light.

So what exactly to make of this excess is somewhat unclear. Hopefully CMS can weigh in soon!

## Excesses 2-4: CMS’s Taus; Vector-Like-Leptons and TauTau Resonance(s)

Paper 1 : https://cds.cern.ch/record/2803736

Paper 2: https://cds.cern.ch/record/2803739

Many of the models seeking to explain the flavor anomalies seen by LHCb predict new particles that couple preferentially to tau’s and b-quarks. These two separate CMS analyses look for particles that decay specifically to tau leptons.

In the first analysis they look for pairs of vector-like-leptons (VLL’s) the lightest particle predicted in one of the favored models to explain the flavor anomalies. The VLL’s are predicted to decay into tau leptons and b-quarks, so the analysis targets events which have at least four b-tagged jets and reconstructed tau leptons. They trained a machine learning classifier to separate VLL’s from their backgrounds. They see an excess of events at high VLL classification probability in the categories with 1 or 2 reconstructed tau’s, with a significance of 2.8 standard deviations.

In the second analysis they look for new resonances that decay into two tau leptons. They employ a sophisticated ’embedding’ technique to estimate the large background of Z bosons decaying to tau pairs by using the decays of Z bosons to muons. They see two excesses, one at 100 GeV and one at 1200 GeV, each with a significances of around 3-sigma. The excess at ~100 GeV could also be related to another CMS analysis that saw an excess of diphoton events at ~95 GeV, especially given that if there was an additional Higgs-like boson at 95 GeV  diphoton and ditau would be the two channels it would likely first appear in.

While the statistical significances of these excess are not quite as high as the first one, meaning it is more likely they are fluctuations that will disappear with more data, their connection to other anomalies is quite intriguing.

## Excess 4: CMS Paired Dijet Resonances

Paper: https://cds.cern.ch/record/2803669

Often statistical significance doesn’t tell the full story of an excess. When CMS first performed its standard dijet search on Run2 LHC data, where one looks for a resonance decaying to two jets by looking for bumps in the dijet invariant mass spectrum, they did not find any significant excesses. But they did note one particular striking event, which 4 jets which form two ‘wide jets’, each with a mass of 1.9 TeV and the 4 jet mass is 8 TeV.

This single event seems very likely to occur via normal Standard Model QCD which normally has a regular 2-jet topology. However a new 8 TeV resonance which decayed to two intermediate particles with masses of 1.9 TeV which then each decayed to a pair of jets would lead to such a signature. This motivated them to design this analysis, a new search specifically targeting this paired dijet resonance topology. In this new search they have now found a second event with very similar characteristics. The local statistical significance of this excess is 3.9-sigma, but when one accounts for the many different potential dijet and 4-jet mass combinations which were considered in the analysis that drops to 1.6-sigma.

Though 1.6-sigma is relatively low, the striking nature of these events is certainly intriguing and warrants follow up. The Run-3 will also bring a slight increase to the LHC’s energy (13 -> 13.6 TeV) which will give the production rate of any new 8 TeV particles a not-insignificant boost.

## Conclusions

The safe bet on any of these excesses would probably be that it will disappear with more data, as many excesses have done in the past. And many particle physicists are probably wary of getting too excited after the infamous 750 GeV diphoton fiasco in which many people got very excited (and wrote hundreds of papers about) a about a few-sigma excess in CMS + ATLAS data that disappeared as more data was collected. All of theses excesses are for analyses only performed by a single experiment (ATLAS or CMS) for now, but both experiments have similar capabilities so it will be interesting to see what the counterpart has to say for each excess once they perform a similar analysis on their Run-2 data. At the very least these results add some excitement for the upcoming LHC Run-3–the LHC collisions are starting up again this year after being on hiatus since 2018.

CERN Courier Article “Dijet excess intrigues at CMS”

Background on the imfamous 750 GeV diphoton excess, Physics World Article “And so to bed for the 750 GeV bump

Background on the LHCb flavor anomalies, CERN Courier “New data strengthens RK flavour anomaly

## A hint of CEvNS heaven at a nuclear reactor

Title : “Suggestive evidence for Coherent Elastic Neutrino-Nucleus Scattering from reactor antineutrinos”

Authors : J. Colaresi et al.

Neutrinos are the ghosts of particle physics, passing right through matter as if it isn’t there. Their head-on collisions with atoms are so rare that it takes a many-ton detector to see them. Far more often though, a neutrino gives a tiny push to an atom’s nucleus, like a golf ball glancing off a bowling ball. Even a small detector can catch these frequent scrapes, but only if it can pick up the bowling ball’s tiny budge. Today’s paper may mark the opening of a new window into these events, called “coherent neutrino-nucleus scattering” or CEvNS (pronounced “sevens”), which can teach us about neutrinos, their astrophysical origins, and the even more elusive dark matter.

#### A scrape with a ghost in a sea of noise

CEvNS was first measured in 2017 by COHERENT at a neutron beam facility, but much more data is needed to fully understand it. Nuclear reactors produce far more neutrinos than other sources, but they are even less energetic and thus harder to detect. To find these abundant but evasive events, the authors used a detector called “NCC-1701” that can count the electrons knocked off a germanium atom when a neutrino from the reactor collides with its nucleus.

Unfortunately, a nuclear reactor produces lots of neutrons as well, which glance off atoms just like neutrinos, and the detector was further swamped with electronic noise due to its hot, buzzing surroundings. To pick out CEvNS from this mess, the researchers found creative ways to reduce these effects: shielding the detector from as many neutrons as possible, cooling its vicinity, and controlling for environmental variables.

#### An intriguing bump with a promising future

After all this work, a clear bump was visible in the data when the reactor was running, and disappeared when it was turned off. You can see this difference in the top and bottom of Fig. 1, which shows the number of events observed after subtracting the backgrounds, as a function of the energy they deposited (number of electrons released from germanium atoms).

But measuring CEvNS is such a new enterprise that it isn’t clear exactly what to look for – the number of electrons a neutrino tends to knock off a germanium atom is still uncertain. This can be seen in the top of Fig. 1, where the model used for this number changes the amount of CEvNS expected (solid vs dashed line).

Still, for a range of these models, statistical tests “moderately” to “very strongly” confirmed CEvNS as the likely explanation of the excess events. When more data accumulates and the bump becomes clearer, NCC-1701 can determine which model is correct. CEvNS may then become the easiest way to measure neutrinos, since detectors only need to be a couple feet in size.

Understanding CEvNS is also critical for finding dark matter. With dark matter detectors coming up empty, it now seems that dark matter hits atoms even less often than neutrinos, making CEvNS an important background for dark matter hunters. If experiments like NCC-1701 can determine CEvNS models, then dark matter searches can stop worrying about this rain of neutrinos from the sky and instead start looking for them. These “astrophysical” neutrinos are cosmic messengers carrying information about their origins, from our sun’s core to supernovae.

This suggestive bump in the data of a tiny detector near the roiling furnace of a nuclear reactor shows just how far neutrino physics has come – the sneakiest ghosts in the Standard Model can now be captured with a germanium crystal that could fit in your palm. Who knows what this new window will reveal?

Ever-Elusive Neutrinos Spotted Bouncing Off Nuclei for the First Time” – Scientific American article from the first COHERENT detection in 2017

Hitting the neutrino floor” – Symmetry Magazine article on the importance of CEvNS to dark matter searches

Local nuclear reactor helps scientists catch and study neutrinos” – Phys.org story about these results

## Exciting headways into mining black holes for energy!

Based on the paper Penrose process for a charged black hole in a uniform magnetic field

It has been over half a century since Roger Penrose first theorized that spinning black holes could be used as energy powerhouses by masterfully exploiting the principles of special and general relativity [1, 2]. Although we might not be able to harness energy from a black hole to reheat that cup of lukewarm coffee just yet, with a slew of amazing breakthroughs [4, 5, 6], it seems that we may be closer than ever before to making the transition from pure thought experiment to finally figuring out a realistic powering mechanism for several high-energy astrophysical phenomena. Not only can there be dramatic increases in the energies of radiated particles using charged, spinning black holes as energy reservoirs via the electromagnetic Penrose process rather than neutral, spinning black holes via the original mechanical Penrose process, the authors of this paper also demonstrate that the region outside the event horizon (see below) from which energy can be extracted is much larger in the former than the latter. In fact, the enhanced power of this process is so great, that it is one of the most suitable candidates for explaining various high-energy astrophysical phenomena such as ultrahigh-energy cosmic rays, particles [7, 8, 9] and relativistic jets [10, 11].

Stellar black holes are the final stages in the life cycle of stars so massive that they collapse upon themselves, unable to withstand their own gravitational pull. They are characterized by a point-like singularity at the centre where a complete breakdown of Einstein’s equations of general relativity occurs, and surrounded by an outer event horizon, within which the gravitational force is so strong that not even light can escape it. Just outside the event horizon of a rotating black hole is a region called the ergosphere, bounded by an outer stationary surface, within which space-time is dragged along inexorably with the black hole via a process called frame-dragging. This effect predicted by Einstein’s theory of general relativity, makes it impossible for an object to stand still with respect to an outside observer.

The ergosphere has a rather curious property that makes the word-line (the path traced in $4-$dim space-time) of a particle or observer change from being time-like outside the static surface to being space-like inside it. In other words, the time and angular coordinates of the metric swap places! This leads to the existence of negative energy states of particles orbiting the black hole with respect to observer at infinity [2, 12, 13]. It is this very property that enables the extraction of rotational energy from the ergosphere as explained below.

According to Penrose’s calculations, if a massive particle that falls into the ergosphere were to split into two, the daughter who gets a kick from the black hole, would be accelerated out with a much higher positive energy (upto $20.7$ percent higher to be exact) than the in-falling parent, as long as her sister is left with a negative energy. While it may seem counter-intuitive to imagine a particle with negative energy, note that no laws of relativity or thermodynamics are actually broken. This is because the observed energy of any particle is relative, and depends upon the momentum measured in the rest frame of the observer. Thus, a positive kinetic energy of the daughter particle left behind would be measured as negative by an observer at infinity [3].

In contrast to the purely geometric mechanical Penrose process, if one now considers black holes that possess charge as well as spin, a tremendous amount of energy stored in the electromagnetic fields can be tapped into, leading to ultra high energy extraction efficiencies. While there is a common misconception that a charged black hole tends to neutralize itself swiftly by attracting oppositely charged particles from the ambient medium, this is not quite true for a spinning black hole in a magnetic field (due to the dynamics of the hot plasma soup in which it is embedded). In fact in this case, Wald [14] showed that black holes tend to charge up till they reach a certain energetically favourable value. This value plays a crucial role in the amount of energy that can be delivered to the outgoing particle through the electromagnetic Penrose process. The authors of this paper explicitly locate the regions from which energy can be extracted and show that these are no longer restricted to the ergosphere, as there are a whole bunch of previously inaccessible negative energy states that can now be mined. They also find novel disconnected, toroidal regions not coincident with the ergosphere that can trap the negative energy particles forever (refer to Fig.1)! The authors calculate the effective coupling strength between the black hole and charged particles, a certain combination of the mass and charge parameters of the black hole and charged particle, and the external magnetic field. This simple coupling formula enables them to estimate the efficiency of the process as the magnitude of the energy boost that can be delivered to the outgoing particle is directly dependent on it. They also find that the coupling strength decreases as energy is extracted, much the same way as the spin of a black hole decreases as it loses energy to the super-radiant particles in the mechanical analogue.

While the electromagnetic Penrose process is the most favoured astrophysically viable mechanism for high energy sources and phenomena such as quasars, fast radio bursts, relativistic jets etc., as the authors mention “Just because a particle can decay into a trapped negative-energy daughter and a significantly boosted positive-energy radiator, does not mean it will do so..” However, in this era of precision black hole astrophysics, state-of-the-art observatories, the Event Horizon Telescope capable of capturing detailed observations of emission mechanisms in real time, and enhanced numerical and scientific methods at our disposal, it appears that we might be on the verge of detecting observable imprints left by the Penrose process on black holes, and perhaps tap into a source of energy for advanced civilisations!

References