Measurement of the isolated photon cross section in proton-antiproton collisions at DØ

At the heart of Fermilab's two collider detectors, literally millions of particle collisions occur each second. Of these collisions, the vast majority are governed by strong interaction, the strongest of the four forces in nature of which we know. In most of these collisions, jets are produced, which are the signature of scattering quarks and gluons - the fundamental constituents of protons and neutrons. With such an enormous number of such collisions, one would think that the strong force is the most precisely measured of the forces, but this is not true. Quantum Chromodynamics (QCD) is the theory that describes the strong interactions of the quarks and gluons. While this theory is well understood and many excellent measurements exist, the complexity (and sometimes ambiguity) of the calculations and the details of the measurements preclude simple interpretation of the data.

Recent investigation of photon production in DØ collaboration is helping to sort out some of the confusion. Photons, which are the carrier of the electromagnetic force (perfectly studied by physicists), do not suffer from the same calculational and measurement difficulties of jets, allowing physicists to peer deeply inside the collision. Unlike jets, which are only a distant cousin of the quarks and gluons, that actually undergo the collision, a photon measurement allows a unique glimpse at the heart of the collision.

Further, a photon measurement helps to clarify just how the energy inside the proton is shared between quarks and gluons. Our understanding of how the energy is shared among gluons has been a limitation when interpreting many jet-production events. Photon measurements of this type are helping to sort out this mystery.
Figure 1: The fact that a photon does not interact after the hard scatter gives unique insight into the underlying (and interesting) high-energy quark-gluon collision. The photonic events registered in the DØ calorimeter, shown on the right side of the picture, schematically can be presented by the Feynman diagram on the left side. (Click on image for larger version.)
The fact that a photon does not interact after the hard scatter gives unique insight into the underlying (and interesting) high-energy collision.

Finally, the produced photons may be an important sign of a new particles and physics. Thus, first of all, it is necessary to understand the "conventional" sources of photons.

Unfortunately, this measurement is complicated by the presence of the energetic neutral measons, produced in the core of hadronic jets, that mimic the photon signal. But the selection criteria (including the photon isolation) and the identification method we have found, allows substantially to get rid off the background events and register photon signal with a reasonable accuracy.

In DØ we have measured a probability (cross section) of the photon production over a wide range of the photon transverse momentum pT, 23 < pT < 300 GeV, that significantly extends previous analogous measurements (Figure 2). One can see from Figure 2 that in the presented pT interval the photon cross section falls by more than five orders of magnitude. The uncertainties of our measurement are comparable with existing theoretical ones. We concluded that the found photon cross section agree with the theoretical predictions in the whole pT interval.

Figure 2: The inclusive cross section for the production of isolated photons as a function of photon pT. The results from the theoretical calculations are shown as solid line. (Click on image for larger version.)

We had just five events with pT > 300 GeV (which were not used in this analysis) but due to increasing Tevatron luminosity we hope to have much more statistics in the next 2-3 years. It would allow us to study the region up to pT = 500-600 GeV and to check as predictions of the "standard" theory as to look for possible "traces" of new physics.

The paper describing these results has been published as Phys. Lett. B639, 151 (2006).

If you have any questions about this analysis, please feel free to contact the primary authors Dmitry Bandurin and Michael Begel.

November 24, 2005