For the first time at the Large Hadron Collider, researchers with the CMS experiment have observed W and Z bosons and TeV-scale jets alongside a proton surviving a collision intact, reviving a previously obscured area of study. This observation overcomes a significant technical hurdle, high pileup, allowing exploration of hard diffraction, a phenomenon where high-energy collisions leave one proton unscathed.
Before quantum chromodynamics fully explained the strong interaction, physicists sought to understand hadron behavior; in 1968, G. Veneziano discovered a formula linking hadron resonances to high-energy behavior predicted by Regge theory, a connection whose deeper meaning later emerged. CMS addressed the challenge with the Precision Proton Spectrometer, enabling studies even during standard high-pileup running.
PPS Detects Intact Protons at LHC Amidst High Pileup
The CMS experiment recorded the first observation of W and Z bosons produced alongside intact protons following collisions at 13.6 TeV, a result previously obscured by the challenges of high pileup. This achievement allows physicists to revisit hard diffraction, specifically investigating how often high-energy collisions leave one proton unscathed after the interaction.
The analysis hinges on matching forward-proton kinematics to central collision signatures, a technique CMS addressed with the Precision Proton Spectrometer, enabling studies even during standard high-pileup running. This renewed exploration of diffractive processes connects to theoretical work dating back to 1968, when G. While the formula’s deeper meaning wasn’t fully understood at the time, it now provides a framework for interpreting these observations and understanding the exchange mechanisms at play during proton collisions.
The current findings build upon earlier attempts to study diffraction at the Tevatron, where roughly an order of magnitude fewer hard diffractive events were observed compared to predictions derived from HERA data, a discrepancy that continues to puzzle physicists. The complexity of modern LHC collisions, with over 60 interactions occurring per bunch crossing, presented a significant hurdle; an unrelated proton could be incorrectly associated with the primary hard scattering event. CMS overcame this by carefully comparing the observed rate of events with a tagged proton to the expected accidental background, establishing a correlation between the forward proton and the central interaction.
An example of such an event shows two nearly back-to-back high-energy jets, with a combined mass of approximately 2 TeV, in association with a tagged intact proton. “Observation of the production of high-p T jets or electroweak bosons with an intact forward proton in pp collisions at √s = 13.6 TeV” is the CMS Physics Analysis Summary (SMP-26-015). While identifying a forward proton confirms its presence, determining its origin, whether from a colorless exchange or proton dissociation, remains an area of ongoing investigation, as the Regge picture used in current simulations is only partially constrained.
Regge Theory & Diffractive Processes Revived by CMS Data
These are the first W and Z bosons with tagged protons observed at the LHC, and the first proton-tagged multijet events with invariant masses extending to the TeV scale. This achievement allows for a more detailed examination of how frequently high-energy collisions preserve a proton’s integrity, a key question in understanding the strong interaction.
Prior to the establishment of quantum chromodynamics, physicists relied on Regge theory to interpret hadron behavior, a framework originating with the 1968 discovery by G. This formula, initially a mathematical curiosity, later revealed a connection to the exchange of particles, Reggeons, during hadron scattering, describing how momentum is transferred while potentially leaving one hadron intact.
The current CMS data provides a new lens through which to test and refine these early theoretical concepts, bridging a gap between pre-QCD physics and modern collider experiments. This unexpected result suggests that current models may not fully capture the dynamics of diffractive processes, prompting a re-evaluation of the underlying mechanisms and potentially leading to a more complete understanding of particle interactions.
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