[Paper Review] A Heavy Metal Path to New Physics

Marco Drewes, A. Giammanco+3 more|arXiv (Cornell University)|Oct 1, 2018

Particle physics theoretical and experimental studies4 citations

TL;DR

The paper proposes using heavy ion collisions at the LHC as a superior environment for discovering New Physics with displaced vertices, leveraging lower instantaneous luminosity for looser triggers and reduced pile-up to enhance sensitivity to long-lived particles. It demonstrates that for heavy neutrinos in the Neutrino Minimal Standard Model, heavy ion collisions can offer higher sensitivity per unit time than proton-proton collisions.

ABSTRACT

We show that heavy ion collisions at the LHC provide a promising environment to search for signatures with displaced vertices in well-motivated New Physics scenarios. The lower instantaneous luminosity in heavy ion collisions allows to operate the LHC experiments with very loose triggers. For scenarios in which long-lived particles are produced in the decay of light particles, this can increase the number of observable events by several orders of magnitude. If ions lighter than Pb are used, as it is currently discussed in the heavy ion community for unrelated reasons, this can lead to a higher sensitivity per time of running than in pp collisions. We illustrate that explicitly for heavy neutrinos in the Neutrino Minimal Standard Model. Another advantage of heavy ion collisions is the fact that there is no pile up, i.e., the average number of simultaneous interactions per bunch crossing is well below unity. This entirely removes the problem of mis-identifying the location of the primary vertex, which may be the key to trespass the systematics wall due to background uncertainties in the cases where background contamination mostly comes from SM particles that originate from different parts of the interaction region. This provides strong motivation to further explore the possibility to search for New Physics in heavy ion collisions.

Motivation & Objective

• To explore the potential of heavy ion collisions at the LHC for discovering New Physics with displaced vertices.
• To address the challenge of background uncertainties in displaced vertex searches, particularly from misidentified primary vertices due to pile-up in proton-proton collisions.
• To demonstrate that lower instantaneous luminosity in heavy ion collisions enables looser triggers, significantly increasing the number of observable events for long-lived particles.
• To show that using lighter ions than lead could further improve sensitivity per unit time compared to pp collisions.
• To highlight the absence of pile-up in heavy ion collisions as a key advantage in reducing systematics from background contamination.

Proposed method

• Utilize the lower instantaneous luminosity in heavy ion collisions to enable very loose trigger thresholds, increasing the acceptance for events with displaced vertices.
• Model the production and decay of long-lived particles, such as heavy neutrinos in the Neutrino Minimal Standard Model, within the heavy ion environment.
• Simulate the impact of reduced pile-up (average <1 interaction per bunch crossing) on vertex reconstruction accuracy and background suppression.
• Compare the sensitivity of heavy ion collisions to proton-proton collisions by evaluating the number of observable events per unit time.
• Assess the signal-to-background ratio improvement due to the clean interaction environment and precise primary vertex identification in heavy ion collisions.

Experimental results

Research questions

• RQ1 Can heavy ion collisions at the LHC significantly enhance the discovery reach for long-lived particles with displaced vertices compared to proton-proton collisions?
• RQ2 To what extent does reduced pile-up in heavy ion collisions mitigate systematic uncertainties from misidentified primary vertices?
• RQ3 How does the use of lighter ions than lead affect the sensitivity to New Physics signals in displaced vertex searches?
• RQ4 What is the impact of looser triggers in heavy ion collisions on the number of observable events for long-lived particles?
• RQ5 Can the Neutrino Minimal Standard Model serve as a concrete benchmark for demonstrating the advantages of heavy ion collisions in New Physics searches?

Key findings

• The lower instantaneous luminosity in heavy ion collisions enables the use of very loose triggers, increasing the number of observable events for long-lived particles by several orders of magnitude.
• The absence of pile-up in heavy ion collisions eliminates the problem of mis-identifying the primary vertex, which is a major source of background uncertainty in proton-proton collisions.
• For heavy neutrinos in the Neutrino Minimal Standard Model, heavy ion collisions can achieve higher sensitivity per unit time than proton-proton collisions.
• Using ions lighter than lead—currently under discussion in the heavy ion community—can further enhance sensitivity per unit time compared to Pb-Pb collisions.
• The clean environment of heavy ion collisions significantly reduces background contamination from Standard Model particles originating from different parts of the interaction region.