Lawrence Livermore National Laboratory



Douglas M. Wright (15-ERD-062)

Abstract

This research project has helped establish a leading role for our team in the analysis of data collected at the Large Hadron Collider (LHC) located near Geneva, Switzerland, as part of an innovative new approach that is capable of revealing new physics beyond the standard theory of particle physics. We created and published an analysis that is the first to show evidence for the rare process upon which this new approach is based. As key members of a detector upgrade effort to further exploit this new technique, we demonstrated the necessary performance of the precision timing system that we developed to enable a further leap in sensitivity to discovery of new particles with data currently being collected at the Large Hadron Collider.

Background and Research Objectives

The Nobel-prize winning discovery of the Higgs boson (CMS 2012) at the Large Hadron Collider (LHC) has raised more compelling questions than it has answered (Cho 2007). While the Higgs appears to be the long-sought mechanism that provides mass to all fundamental particles in the universe, without additional new fundamental particles or forces, the theory suffers from a number of catastrophic problems. For decades, theorists have developed various solutions to these problems. However, experimental evidence for any of the proposed new phenomena has been extremely difficult to acquire. A common feature foremost among these models (Belanger 1992, Bell 2009, Royon 2011) is an enhancement to the production of W boson pairs, which is intimately connected to the Higgs mechanism and the behavior of the theory for W boson scattering at high energy.

Now that a standard model Higgs boson particle (and no other new particle) has been observed, there is much greater interest in non-supersymmetry theories like composite Higgs and gravity-motivated models such as Randall Sundrum Higgs, Kaluza-Klein gravitons, and radion/dilaton models, all of which produce a standard Higgs with other higher-energy resonances. While the energy and intensity of the LHC continue to improve, the background associated with these gains in accelerator performance threaten to outstrip any increase in signal for new physics. Our project exploited a new experimental technique, pioneered by Livermore, that is capable of detecting evidence of new physics from W boson pairs taking advantage of a dramatic improvement in the ability to reject the obscuring background (Albrow 2005).

Scientific Approach and Accomplishments

There are hundreds of conventional analyses to search for evidence of new physics at the LHC; they all face two severe practical problems: (1) the extremely high intensity of the proton beams result in multiple simultaneous interactions (up to 50 or more occurring at a rate of 40 MHz, i.e., about one billion interactions per second), which are effectively impossible to separate, and (2) the protons involved in the collision break up, thereby producing additional background debris, as well as dramatically increasing the uncertainty in the initial condition of the collision. Livermore was a founding member of a small team of particle physicists that developed an innovative method to address both of these major problems (Albrow 2005).

The approach is to focus on a subset of collisions in which the beam protons interact only by means of high-energy photons. In such an interaction the protons remain intact but lose a significant amount of energy. This energy can then appear in the form of new particles, which decay and are observed by the main central detector. To detect the outgoing protons, we placed small tracking and timing devices inside the LHC beam pipe at approximately 220 meters on both sides of the central collision point. With precise tracking and timing of the outgoing protons, we can determine the exact initial conditions of the interaction and isolate the origin of the collision. This provides a tremendous improvement in the ability to reject background. Recent estimates show that this proton-tagging capability enables analyses at the LHC to achieve the sensitivity necessary to discover new physics in the mass range of a few tera-electronvolts (Fichet 2014).

Livermore is a leading member of the hardware project to upgrade the Compact Muon Solenoid (CMS) experiment to implement precision proton detectors in the LHC beam pipes. The upgrade installation began in 2016 and is nearly complete. Livermore developed and deployed a reference clock system that is able to synchronize the timing detectors that are separated by 500 m with a precision better than one picosecond. The hardware goal for this LDRD project was to commission the precision clock system and measure its performance. Previously we measured the intrinsic timing stability of our clock hardware to be 30 femtoseconds. During this project, we completed a three-week long test that verified our clock system remains synchronized with the LHC accelerator reference signal as it progresses through various frequency changes associated with the normal cycle of the proton beams for physics collisions. The final clock system was then operated successfully in the LHC tunnel. During 2016 and 2017 we diagnosed and resolved issues due to radiation damage to the low-voltage power supplies for the clock. We also developed a diagnostic technique to measure and compare the high-speed signals of our clock and other timing components directly in the LHC tunnel. With this diagnostic, we demonstrated that our clock remains stable during the data-acquisition initialization process, in which various external components can introduce spurious timing signals. Once the high-speed digitizer electronics were completed by our collaborators in mid-2017, we demonstrated the full system performance by comparing a standard signal injected simultaneously on two detector channels at the same location. Using our reference clock, we demonstrated that the timing precision was limited only by the digitizer itself, which was the expected 20 picoseconds.

The proton tagging detector system was installed in stages beginning in 2016. A complete system that includes timing detectors and digitizer electronics was available towards the end of 2017. Data were collected throughout the various stages and is currently ongoing. In this project, we maximized our physics output by executing physics analysis appropriate to the data collected in each stage. As a consequence, we lead the particle physics community with the best sensitivity to new physics from W boson pair production.

When this project began, we took the lead in the analysis of previously unpublished data collected at an LHC energy of 8 TeV without proton tagging detectors. This analysis selects W boson pairs from their decay to an electron and muon pair and requires the origin of the decay to be sufficiently isolated from all other particles in the event. This data set was four times larger and at a higher energy than our previously published analysis. Furthermore, the background from simultaneously occurring events was doubled. To accommodate these conditions, we developed a number of major improvements to the analysis. We expanded the use of control samples extracted from the data itself to estimate the amount of background in and composition of the final signal selection. We developed an analysis of electron pairs, in addition to muon pairs, to estimate the signal contribution from protons that break up after the collision but do not produce detectable debris in the central detector. We found that the isolation requirement did not match well between data and simulations, so we developed a correction scheme for the simulations using controls sample from the data.

Our final published result was 13 observed events with an expected background of 3.5 ± 0.5 events (CMS 2016). Figure 1 shows the tracks observed in our detector for one of these candidate events. This result is the first evidence for the rare process of W boson pairs produced from intact protons.


Figure 1.
Figure 1. Large Hadron Collider proton collision event from our analysis that provides evidence for the rare process: W pair production from intact protons. The isolated red and blue tracks are identified as a muon and electron.

Figure 2 shows the distribution of the mass of the particle state that produced the observed data events and the expected signal and background contributions. The light blue histogram shows the expected contribution from standard physics. New physics would appear as an excess in both the low- and high-mass region. An excess in the high-mass region, where the standard expectation is extremely small, would be an unambiguous indication of new physics. From our analysis, we have set the world's best limit on the presence of new particles from this mode and it is about three times better than our previously published result.


Figure 2.
Figure 2. Distribution of the mass of the particle state that produced the muon and electron detected in the 8-TeV data set. Our data is the first to show evidence for the standard model production of W bosons in this rare process (theory prediction is shown as light blue).

The next available data set to extend our new physics search was collected in 2016 at the substantially higher LHC energy of 13 TeV, where the simultaneous background from multiple events was two times worse than the previous data set at 8 TeV. Part of this data set contains tracking information for the outgoing proton, but does not have any timing information. Because the background conditions are worse and there is only partial proton tagging information, it is not yet known which approach will produce the best sensitivity to new physics. Because of this uncertainty we began two related analyses: one with and one without proton tracking information. When comparing data to simulation for the analysis without tracking information, we found more data events than expected, which is shown in Figure 3. Preliminary investigations show that this is likely an upward fluctuation in our background processes, not a new physics signal. More studies need to be performed to confirm this with control samples in the data and improved simulations. In the second analysis, we observed for the first time W boson pair production with identified outgoing protons in our tracking detectors. We found three such data events that pass all our selection criteria. To determine the expected background and contribution of signal from the standard theory requires significant additional effort and depends on detailed simulations of the outgoing proton, which have not yet been fully implemented.


Figure 2.
Figure 3. Momentum of the particle state that produced the muon and electron detected in the 13-TeV data set without proton tracking information. Data events are shown as points with error bars, predictions from simulations are shown in colored histograms. When comparing data to simulations for the analysis without tracking information, we found more data events than expected, probably indicating an upward fluctuation in our background processes.

Impact on Mission

This project enabled Livermore to be at the forefront of an important new approach in discovery science within the highest priority mission of the DOE Office of High Energy Physics and provides us with the opportunity to make a discovery as important as that of the Higgs boson itself. This also establishes our reputation to participate in future major collider projects such as the High Luminosity upgrade to the LHC and the International Linear Collider.

A highly visible role at the frontier of particle physics attracts and retains outstanding scientific staff and allows us to develop core capabilities in both hardware and data analysis that have direct benefits to Livermore missions. Significant recent examples from personnel on this project are: Department of Defense, Defense Threat Reduction Agency awarding Livermore leadership of their entire active interrogation experimental program and Department of Homeland Security, Domestic Nuclear Detection Office recruiting Livermore to lead a technical effort to address major performance issues with a muon tomography system adapted from high-energy physics detectors. Both of these efforts use expertise and software tools that we developed directly for our particle physics basic science projects.

Conclusion

We are collecting data now with a fully functional proton tagging system. We are leading the analysis of W boson pair production with this data and will continue to produce world's best sensitivity to new physics from this channel. We anticipate that our steady participation in this physics and ongoing upgrades of the proton tagging detectors will result in funding from DOE Office of High Energy Physics (OHEP). We continue to be invited to the OHEP review of LHC physics (Energy Frontier), which are held every three years. Because of our substantial progress from this LDRD project, we are in a strong position for the upcoming review in 2018.

References

Albrow, M., et al. 2005. “FP420: An R&D Proposal to Investigate the Feasibility of Installing Proton Tagging Detectors in the 420m Region at LHC.” CERN-LHCC-2005-025.

Belanger, G., and F. Boudjema. 1992. “Gamma–Gamma to W+W- and Gamma–Gamma to ZZ as Tests of Novel Quartic Couplings.” Phys. Lett. B 288, 210.

Bell, P. J. 2009. “Quartic Gauge Couplings and the Radiation Zero in pp to l nu Gamma-Gamma Events at the LHC.” Eur. Phys. J. C 64, 25.

Cho, A. 2007. “Physicists Nightmare Scenario: The Higgs and Nothing Else.” Science 315 (5819):1657–1658.

CMS Collaboration. 2012. “Observation of a New Boson at a Mass of 125 GeV with the CMS Experiment at the LHC.” Phys. Lett. B 716, 30.

CMS Collaboration. 2016. “Evidence for Exclusive Gamma–Gamma to W+W- Production and Constraints on Anomalous Quartic Gauge Couplings in pp Collisions at vs = 7 and 8 TeV.” J. High Energ. Phys. 2016:119. LLNL-JRNL-704026.

Fichet, S., and G. von Gersdorff. 2014. “Anomalous Gauge Couplings from Composite Higgs and Warped Extra Dimensions.” J. High Energ. Phys. 2014:1403.

Royon, C., et al. 2011. “Anomalous Quartic and Triple Gauge Couplings in Gamma-Induced Processes at the LHC.” Am. Inst. Phys. Conf. Proc. 1350, 140.

Publications and Presentations

CMS Collaboration. 2016. “Evidence for Exclusive Gamma–Gamma to W+W- Production and Constraints on Anomalous Quartic Gauge Couplings in pp Collisions at vs = 7 and 8 TeV.” J. High Energ. Phys. 2016:119. LLNL-JRNL-704026.