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Quark Excitement: Is there anything smaller?
30 May 2012

Mankind has forever sought to determine the most fundamental components of matter. From the atom to the nucleus to the proton and neutron, and finally to the quark, we have asked each step of the way "Is this it or is there something inside?"

ATLAS physicists have just taken another step toward tackling that very question by publishing the results of a search for new kinds of particles decaying into a jet (a spray of hadronic particles) and a photon.

The Physical Review Letters article provides the world's best upper limits to date on the probability of producing such particles, including excited states of quarks.

A recent ATLAS Blog posting explains that, as in the case of atoms, if a particle can be excited then it is necessarily composed of smaller pieces. If the LHC were able to create excited quarks, we should observe them with ATLAS as they emit photons of light and return to being regular quarks.

Although no such excited states were found, the ATLAS study has significantly extended previous results obtained at other colliders. In fact, these measurements rule out the existence of signals ten times fainter and excited quarks 2 TeV more massive than earlier studies.

With the 2012 increase in the LHC energy to 8 TeV, and additional increases in the future, we expect to use these and other techniques to extend the reach of our understanding of matter's most fundamental constituents.

Refer to a new ATLAS Blog article describing our technique and this new result in the search for quark substructure.

The upper part of this plot shows a histogram (black dots) of ATLAS data events containing a photon and a jet, organized into bins defined by the mass of the photon-plus-jet pair. The stepped solid line represents a mathematical background function that has been fitted to the ATLAS data. If an exotic particle, having a mass in this range and decaying to a photon and a jet, were produced in the LHC, we would expect a bump to appear in the ATLAS data. Depending on the mass of the new particle and how readily it is created in the LHC, the bump could resemble one of the three indicated coloured peaks representing hypothetical excited quarks (q*) having masses of 0.5, 1.0, and 2.0 TeV. The lower part of this plot shows the statistical significance of the difference between the ATLAS data and the background function in each bin.


This plot compares how a product of quantities proportional to the number of hypothetical excited q* quarks observable in the ATLAS detector through their decays into a photon and a jet (vertical axis) varies as a function of the q* mass (horizontal axis). The black dots show the 95% credibility-level (CL) upper limits on this product measured by ATLAS in 7 TeV proton collision data. The dashed line shows the upper limits that were expected. The blue line describes how a theoretical excited-quark model predicts the product to vary with q* mass. The q* mass at which the blue curve and the ATLAS observed upper limits intersect marks the 95% CL lower limit set by ATLAS on the hypothetical excited quark mass.

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