## CMS catches the top quark running

CMS catches the top quark running

Article : “Running of the top quark mass from proton-proton collisions at √ s = 13 TeV“

Authors: The CMS Collaboration

Reference: https://arxiv.org/abs/1909.09193

When theorists were first developing quantum field theory in the 1940’s they quickly ran into a problem. Some of their calculations kept producing infinities which didn’t make physical sense. After scratching their heads for a while they eventually came up with a procedure known as renormalization to solve the problem.  Renormalization neatly hid away the infinities that were plaguing their calculations by absorbing them into the constants (like masses and couplings) in the theory, but it also produced some surprising predictions. Renormalization said that all these ‘constants’ weren’t actually constant at all! The value of these ‘constants’ depended on the energy scale at which you probed the theory.

One of the most famous realizations of this phenomena is the ‘running’ of the strong coupling constant. The value of a coupling encodes the strength of a force. The strong nuclear force, responsible for holding protons and neutrons together, is actually so strong at low energies our normal techniques for calculation don’t work. But in 1973, Gross, Wilczek and Politzer realized that in quantum chromodynamics (QCD), the quantum field theory describing the strong force, renormalization would make the strong coupling constant ‘run’ smaller at high energies. This meant at higher energies one could use normal perturbative techniques to do calculations. This behavior of the strong force is called ‘asymptotic freedom’ and earned them a Nobel prize. Thanks to asymptotic freedom, it is actually much easier for us to understand what QCD predicts for high energy LHC collisions than for the properties of bound states like the proton.

Now for the first time, CMS has measured the running of a new fundamental parameter, the mass of the top quark. More than just being a cool thing to see, measuring how the top quark mass runs tests our understanding of QCD and can also be sensitive to physics beyond the Standard Model. The top quark is the heaviest fundamental particle we know about, and many think that it has a key role to play in solving some puzzles of the Standard Model. In order to measure the top quark mass at different energies, CMS used the fact that the rate of producing a top quark-antiquark pair depends on the mass of the top quark. So by measuring this rate at different energies they can extract the top quark mass at different scales.

Top quarks nearly always decay into W-bosons and b quarks. Like all quarks, the b quarks then create a large shower of particles before they reach the detector called a jet. The W-bosons can decay either into a lepton and a neutrino or two quarks. The CMS detector is very good at reconstructing leptons and jets, but neutrinos escape undetected. However one can infer the presence of neutrinos in an event because we know energy must be conserved in the collision, so if neutrinos are produced we will see ‘missing’ energy in the event. The CMS analyzers looked for top anti-top pairs where one W-boson decayed to an electron and a neutrino and the other decayed to a muon and a neutrino. By using information about the electron, muon, missing energy, and jets in an event, the kinematics of the top and anti-top pair can be reconstructed.

The measured running of the top quark mass is shown in Figure 2. The data agree with the predicted running from QCD at the level of 1.1 sigma, and the no-running hypothesis is excluded at above 95% confidence level. Rather than being limited by the amount of data, the main uncertainties in this result come from the theoretical understanding of the top quark production and decay, which the analyzers need to model very precisely in order to extract the top quark mass. So CMS will need some help from theorists if they want to improve this result in the future.

1. “The Strengths of Known Forces” https://profmattstrassler.com/articles-and-posts/particle-physics-basics/the-known-forces-of-nature/the-strength-of-the-known-forces/
2. “Renormalization Made Easy” http://math.ucr.edu/home/baez/renormalization.html
3. “Studying the Higgs via Top Quark Couplings” https://particlebites.com/?p=4718
4. “The QCD Running Coupling” https://arxiv.org/abs/1604.08082
5. CMS Measurement of QCD Running Coupling https://arxiv.org/abs/1609.05331

## The lighter side of Dark Matter

Article title: “Absorption of light dark matter in semiconductors”

Authors: Yonit Hochberg, Tongyan Lin, and Kathryn M. Zurek

 Reference: arXiv:1608.01994

Direct detection strategies for dark matter (DM) have grown significantly from the dominant narrative of looking for scattering of these ghostly particles off of large and heavy nuclei. Such experiments involve searches for the Weakly-Interacting Massive Particles (WIMPs) in the many GeV (gigaelectronvolt) mass range. Such candidates for DM are predicted by many beyond Standard Model (SM) theories, one of the most popular involving a very special and unique extension called supersymmetry. Once dubbed the “WIMP Miracle”, these types of particles were found to possess just the right properties to be suitable as dark matter. However, as these experiments become more and more sensitive, the null results put a lot of stress on their feasibility.

Typical detectors like that of LUX, XENON, PandaX and ZEPLIN, detect flashes of light (scintillation) from the result of particle collisions in noble liquids like argon or xenon. Other cryogenic-type detectors, used in experiments like CDMS, cool semiconductor arrays down to very low temperatures to search for ionization and phonon (quantized lattice vibration) production in crystals. Already incredibly successful at deriving direct detection limits for heavy dark matter, new ideas are emerging to look into the lighter side.

Recently, DM below the GeV range have become the new target of a huge range of detection methods, utilizing new techniques and functional materials – semiconductors, superconductors and even superfluid helium. In such a situation, recoils from the much lighter electrons in fact become much more sensitive than those of such large and heavy nuclear targets.

There are several ways that one can consider light dark matter interacting with electrons. One popular consideration is to introduce a new gauge boson that has a very small ‘kinetic’ mixing with the ordinary photon of the Standard Model. If massive, these ‘dark photons’ could also be potentially dark matter candidates themselves and an interesting avenue for new physics. The specifics of their interaction with the electron are then determined by the mass of the dark photon and the strength of its mixing with the SM photon.

Typically the gap between the valence and conduction bands in semiconductors like silicon and germanium is around an electronvolt (eV). When the energy of the dark matter particle exceeds the band gap, electron excitations in the material can usually be detected through a complicated secondary cascade of electron-hole pair generation. Below the band gap however, there is not enough energy to excite the electron to the conduction band, and so detection proceeds through low-energy multi-phonon excitations, with the dominant being the emission of two back-to-back phonons.

In both these regimes, the absorption rate of dark matter in the material is directly related to the properties of the material, namely its optical properties. In particular, the absorption rate for ordinary SM photons is determined by the polarization tensor in the medium, and in turn the complex conductivity, $\hat{\sigma}(\omega)=\sigma_{1}+i \sigma_{2}$ , through what is known as the optical theorem. Ultimately this describes the response of the material to an electromagnetic field, which has been measured in several energy ranges. This ties together the astrophysical properties of how the dark matter moves through space and the fundamental description of DM-electron interactions at the particle level.

In a more technical sense, the rate of DM absorption, in events per unit time per unit target mass, is given by the following equation:

$R=\frac{1}{\rho} \frac{\rho_{D M}}{m_{A^{\prime}}} \kappa_{e f f}^{2} \sigma_{1}$

• $\rho$ – mass density of the target material
• $\rho_{DM}$ – local dark matter mass density (0.3 GeV/cm3) in the galactic halo
• $m_{A'}$ – mass of the dark photon particle
• $\kappa_{eff}$ – kinetic mixing parameter (in-medium)
• $\sigma_1$ – absorption rate of ordinary SM photons

Shown in Figure 1, the projected sensitivity at 90% confidence limit (C.L.) for a 1 kg-year exposure of semiconductor target to dark photon detection can be almost an order of magnitude greater than existing nuclear recoil experiments. Dependence is shown on the kinetic mixing parameter and the mass of the dark photon. Limits are also shown for existing semiconductor experiments, known as DAMIC and CDMSLite with 0.6 and 70 kg-day exposure, respectively.

Furthermore, in the millielectronvolt-kiloelectronvolt range, these could provide much stronger constraints than any of those that currently exist from sources in astrophysics, even at this exposure. These materials also provide a novel way of detecting DM in a single experiment, so long as improvements are made in phonon detection.

These possibilities, amongst a plethora of other detection materials and strategies, can open up a significant area of parameter space for finally closing in on the identity of the ever-elusive dark matter!

References and further reading:

## Discovering the Tau

This plot [1] is the first experimental evidence for the particle that would eventually be named the tau.

On the horizontal axis is the energy of the experiment. This particular experiment collided electron and positron beams. On the vertical axis is the cross section of a specific event resulting from the electron and positron beams colliding. The cross section is like a probability for a given event to occur. When two particles collide, many many things can happen, each with their own probability. The cross section for an event encodes the probability for that particular event to occur. Events with larger probability have larger cross sections and vice versa.

The collaboration found one event could not be explained by the Standard Model at the time. The event in question looks like:

This event is peculiar because the final state contains both an electron and a muon with opposite charges. In 1975, when this paper was published, there was no way to obtain this final state, from any known particles or interactions.

In order to explain this anomaly, particle physicists proposed the following explanations:

1. Pair production of a heavy lepton. With some insight from the future, we will call this heavy lepton the “tau.”

2. Pair production of charged Bosons. These charged bosons actually end up being the bosons that mediate the weak nuclear force.

The production of tau’s and these bosons are not equally likely though. Depending on the initial energy of the beams, we are more likely to produce one than the other. It turns out that at the energies of this experiment (a few GeV), it is much more likely to produce taus than to produce the bosons. We would say that the taus have a larger cross section than the bosons. From the plot, we can read off that the production of taus, their cross section, is largest at around 5 GeV of energy. Finally, since these taus are the result of pair production, they are produced in pairs. This bump at 5 GeV is the energy at which it is most likely to produce a pair of taus. This plot then predicts the tau to have a mass of about 2.5 GeV.

### References

[1] – Evidence for Anomalous Lepton Production in e+−e− Annihilation. This is the original paper that announced the anomaly that would become the Tau.

[2] – The Discovery of the Tau Lepton. This is a comprehensive story of the discovery of the Tau, written by Martin Perl who would go on to win the 1995 Nobel prize in Physics for its discovery.

[3] – Lepton Review. Hyperphysics provides an accessible review of the Leptonic sector of the Standard Model.

## The early universe in a detector: investigations with heavy-ion experiments

Title: “Probing dense baryon-rich matter with virtual photons”

Reference: https://www.nature.com/articles/s41567-019-0583-8

The quark-gluon plasma, a sea of unbound quarks and gluons moving at relativistic speeds thought to exist at extraordinarily high temperature and density, is a phase of matter critical to our understanding of the early universe and extreme stellar interiors. On the timescale of milliseconds after the Big Bang, the matter in the universe is postulated to have been in a quark-gluon plasma phase, before the universe expanded, cooled, and formed the hadrons we observe today from constituent quarks and gluons. The study of quark matter, the range of phases formed from quarks and gluons, can provide us with insight into the evanescent early universe, providing an intriguing focus for experimentation. Astrophysical objects that are comprised of quarks, such as neutron stars, are also thought to house the necessary conditions for the formation of quark-gluon plasma at their cores. With the accumulation of new data from neutron star mergers, studies of quark matter are becoming increasingly productive and rife for new discovery.

Quantum chromodynamics (QCD) is the theory of quarks and the strong interaction between them. In this theory, quarks and force-carrying gluons, the aptly-named particles that “glue” quarks together, have a “color” charge analogous to charge in quantum electrodynamics (QED). In QCD, the gluon field is often modeled as a narrow tube between two color charges with a constant strong force between them, in contrast with the inverse-square dependence on distance for fields in QED. The pair potential energy between the quarks increases linearly with separation, eventually surpassing the creation energy for a new quark-antiquark pair. Hence, the quarks cannot exist in unbound pairs at low energies, a property known as color confinement. When separation is attempted between quarks, new quarks are instead produced. In particle accelerators, physicists see “jets” of new color-neutral particles (mesons and baryons) in the process of hadronization. At high energies, the story changes and hinges on an idea known as asymptotic freedom, in which the strength of particle interactions decreases with increased energy scale in certain gauge theories such as QCD.

QCD matter is commonly probed with heavy-ion collision experiments and quark-gluon plasma has been produced before in minute quantities at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Lab as well as the Large Hadron Collider (LHC) at CERN. The goal of these experiments is to create conditions similar to those of the early universe or at the center of dense stars — doing so requires intense temperatures and an abundance of quarks. Heavy-ions, such as gold or lead nuclei, fit this bill when smashed together at relativistic speeds. When these collisions occur, the resulting “fireball” of quarks and gluons is unstable and quickly decays into a barrage of new, stable hadrons via the hadronization method discussed above.

There are several main goals of heavy-ion collision experiments around the world, revolving around the study of the phase diagram for quark matter. The first component of this is the search for the critical point: the endpoint of the line of first-order phase transitions. The phase transition between hadronic matter, in which quarks and gluons are confined, and partonic matter, in which they are dissociated in a quark-gluon plasma, is also an active area of investigation. There is additionally an ongoing search for chiral symmetry restoration at finite temperature and finite density. A chiral symmetry occurs when the handedness of the particles remains invariant under a parity transformation, that is, when the sign of a spatial coordinate is flipped. However, in QCD, a symmetric system becomes asymmetric in a process known as spontaneous symmetry breaking. Several experiments are designed to investigate evidence of the restoration of this symmetry.

The HADES (High-Acceptance DiElectron Spectrometer) collaboration is a group attempting to address such questions. In a recent experiment, HADES focused on the creation of quark matter via collisions of a beam of Au (gold) ions with a stack of Au foils. Dileptons, which are bound lepton-antilepton pairs that emerge from the decay of virtual particles, are a key element of HADES’ findings. In quantum field theory (QFT), in which particles are modeled as excitations in an underlying field, virtual particles can be thought of as excitations in the field that are transient due to limitations set by the uncertainty principle. Virtual particles are represented by internal lines in Feynman diagrams, are used as tools in calculations, and are not isolated or measured on their own — they are only exchanged with ordinary particles. In the HADES experiment, the virtual photons that produce dileptons which immediately decouple from the strong force. Produced at all stages of QCD interaction, they are ideal messengers of any modification of hadron properties. They are also thought to contain information about the thermal properties of the underlying medium.

To actually extract this information, the HADES detector utilizes a time-of-flight chamber and ring-imaging Cherenkov (RICH) chamber, which identifies particles using the characteristics of Cherenkov radiation: electromagnetic radiation emitted when a particle travels through a dielectric medium at a velocity greater than the phase velocity of light in that particular medium. The detector is then able to measure the invariant mass, rapidity (a commonly-used substitute measure for relativistic velocity), and transverse momentum of emitted electron-positron pairs, the dilepton of choice. In accelerator experiments, there are typically a number of selection criteria in place to ensure that the machinery is detecting the desired particles and the corresponding data is recorded. When a collision event occurs within HADES, a number of checks are in place to ensure that only electron-positron events are kept, factoring in both the number of detected events and detector inefficiency, while excess and background data is thrown out. The end point of this data collection is a calculation of the four-momenta of each lepton pair, a description of its relativistic energy and momentum components. This allows for the construction of a dilepton spectrum: the distribution of the invariant masses of detected dileptons.

The main data takeaway from this experiment was the observation of an excess of dilepton events in an exponential shape, contrasting with the expected number of dileptons from ordinary particle collisions. This suggests a shift in the properties of the underlying matter, with a reconstructed temperature above 70 MeV (note that particle physicists tend to quote temperatures in more convenient units of electron volts). The kicker comes when the group compares these results to simulated neutron star mergers, with expected core temperatures of 75 MeV. This means that the bulk matter created within HADES is similar to the highly dense matter formed in such mergers, a comparison which has become recently accessible due to multi-messenger signals incorporating both electromagnetic and gravitational wave data.

Practically, we see that HADES’ approach is quite promising for future studies of matter under extreme conditions, with the potential to reveal much about the state of the universe early on in its history as well as probe certain astrophysical objects — an exciting realization!