The very first evidence of top quark production in nucleus-nucleus collisions

Quarks are the heaviest elementary particles known to date. They can be studied in heavy-ion collisions. At the time, it was still unclear whether the LHC was able to sustain collisions between heavy ions at a sufficiently high collision rate, also known as luminosity.

Recently, LHC accelerator experts achieved this rate and surpassed the initial luminosity goals for heavy-ion collisions.

The Compact Muon Solenoid (CMS) Collaboration, a large group of specialists from different institutes worldwide, has recently accumulated the very first proof of top quark production nucleus-nucleus collisions. Experts were able to achieve this rate and surpass the initial luminosity goals for heavy-ion collisions.

As the rate of top quark production depends in significant part on the collision energy, producing these particles in LHC-based heavy-ion collisions seemed challenging.

The LHC set up to give less time to heavy-ion collisions and more to p-p collisions, reflecting the particle physics community’s priorities. For example, in one year, it generally spends one-month producing heavy-ion collisions and six to seven months in p-p crashes.

Finally, heavy-ion collisions produce unmistakably a larger number of particles, more common p-p ones, which can make recognizing particles and examining heavy ion-related data gathered by the LHC extremely challenging. By and large, these variables blocked and hindered the investigation of top quarks in heavy-ion collisions, regardless of whether they were frequently recognized in p-p collisions.

Five years ago, scientists at CERN, the University of Jyväskylä, and Helsinki Institute of Physics predicted the production rate of top quarks in heavy-ion collisions. Despite the relatively low production rate of the LHC, they argued that top quarks could help probe the so-called quark-gluon plasma (QGP).

QGP is a state of matter that is believed to have existed during the universe’s first microsecond of life, which could also reside in the dense core of neutron stars in today’s universe. This state of matter can be recreated in laboratory settings by colliding heavy ions, such as lead (Pb).

Top quarks can be used to explore QGP as well as the distribution of gluons within nuclei. Both uses require different types of collisions, the former symmetric ones (e.g., lead on lead or Pb-Pb) and the latter symmetric and asymmetric ones (e.g., protons on lead or p-Pb).

The LHC collides both symmetric and asymmetric beams. Before it could be applied to QGP and gluon-related studies, scientists confidently needed to prove that top quarks can be detected in nucleus-nucleus collisions.

Scientists reported, “In December 2015, the LHC delivered Pb-Pb collisions with a kinetic energy of 2.51 TeV per nucleon, meaning for the nucleon-nucleon collision, a total (center of mass-energy per nucleon) of 5.02 TeV. This was a big step over Run 1, but the luminosity was still too limited for top-quark study purposes, and, as mentioned before, the heavy-ion running time was only one month. So in short, that dataset was too small to claim evidence for top quark production.”

After this dataset, which was published in 2015, Scientists carried out a series of studies to gather evidence of top quark production in heavy-ion collisions. They measured top quark production in a small reference p-p sample taken in 2015 at the same center-of-mass energy of 5.02 TeV; then they measured it in p-Pb collisions recorded in 2016. Ultimately, they performed their analyses on Pb-Pb collisions.

Scientists noted, “These new Pb-Pb data were accumulated at the very end of Run 2, in 2018, thanks to the ingenuity of our accelerator colleagues, who introduced improvements in the chain from the Pb ion source down to LHC, and the capability of the CMS experiment to record on tape, the full amount of heavy-ion data delivered by LHC. Overall, this resulted in a total accumulated luminosity approximately four times larger than in 2015. The larger data set eventually helped, but by itself, it wouldn’t have been sufficient in case no top quark reconstruction improvements were introduced.”

In a recent study, scientists combined two experimental approaches: one that is affected by QGP and one that is agnostic to it. The first of these methods exploit bottom quarks’ facts (i.e., the lighter versions of top quarks). Bottom quarks can provide hints of top quark production, as the latter almost always decays into the former. On the other hand, the second approach focused exclusively on the study of electrons and muons (i.e., heavier relatives of electrons).

Andrea Giammanco, former coordinator of the Top Quark group of the CMS collaboration, said, “This second method was less sensitive, but it prevented a potential criticism: We have a relatively imprecise knowledge, so far, of how QGP affects the behavior of bottom quarks, and so in principle, the first method might be biased by still unknown effects. As a result of the smallness of the top-quark signal, the large background (e.g., random combinations of unrelated particles or detector-induced processes that mimic the signal), and the complexity of top quark reconstruction, the analysis was designed with a few unique features.”

At first, scientists reoptimized identification algorithms to achieve performances comparable to those attained on p-p collisions, despite the challenges associated with the environment created by Pb-Pb collisions. They especially used advanced machine learning algorithms for the analysis.

Notably, the CMS collaboration was the first to gather measurements that extract top quark signals based on lepton information alone. They also used a new analysis technique that is entirely driven by data to estimate background information carefully.

Giammanco said, “To avoid any human bias, our study was designed following a so-called ‘blind’ analysis procedure, whereby the selection criteria were optimized and fixed first using only a small initial part of the data, before being applied to the full data set. In the end, the agreement of the results from the two approaches between them, with the rate extrapolated from p-p collisions, and with the theoretical expectation, gave us confidence in the first concrete evidence for the production of top quarks in nucleus-nucleus collisions. Crucial to this successful outcome, has also been the precise estimate of the actual luminosity, a task which our team, with the help of the CMS luminosity group, performed with high priority, too.”

Before this study, scientists enabled measurements of various elementary particles with large masses in heavy-ion collisions, such as massive carriers of the electroweak force. There was limited evidence for top quark production in heavy-ion collisions, even if theoretical predictions suggested that they were produced at a sufficiently high rate.

While gathering the first evidence of top quark production in nucleus-nucleus collisions, scientists measured a collision rate aligned with theoretical predictions.

Georgios K. Krintiras, co-coordinator of the Luminosity Group of the CMS collaboration, said, “Actually, our community had never had the chance before for probing such an energy regime (or ‘energy scale’) close to the top quark mass, putting the theory that bounds together nucleons in nuclei, called the ‘strong force,” under stringent tests. Moreover, physics processes used so far, for example, the production of the W and Z bosons and particles of light, the photons, are only sensitive to the properties of QGP integrated over its extremely short lifetime (only a tiny fraction of a second, in technical terms, about seconds). Our paper, following up on recent theory considerations for unveiling the yoctosecond structure of QGP, is just the first step in using the top quark for providing vital novel insights into the time structure of the medium created in heavy-ion collisions.”

“The exceptionally high mass of top quarks we identified sets a new scale for probing the inner structure of the nuclei too, encoded in the so-called nuclear Parton distribution functions (nPDFs). Our current knowledge of how nucleons behave inside a nucleus is limited, mainly because of the lack of data at that scale.”

The recent work by the CMS collaboration could also have important implications for the understanding and search for new physics. Although the research communities investigating heavy ion interactions and new physics are typically unrelated, this first evidence for the production of top quarks in heavy ion interactions has paved the way for a collaboration between these two physics communities.

In the future, scientists are planning to build on their recent findings to conduct additional searches for top quarks in heavy-ion collisions.

Krintiras said, “In our paper, the so-called ‘observed statistical significance’ of the signal amounts to 4.0 units of ‘standard deviations’ (σ), for both methods. In other words, if no top quarks were produced, there would still be a probability of 0.003% (that’s the 4σ level) that the signal would arise from a background fluctuation. We want to decrease this probability further, reaching the higher threshold of 5σ that is considered the standard for declaring observation in our community.”

Journal Reference:
  1. Evidence for top quark production in nucleus-nucleus collisions. Physical Review Letters, (2020). DOI: 10.1103/PhysRevLett.125.222001

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