|The International Linear Collider
Part 3: Designing Detectors for the Unexpected
|Contact: Paul Preuss, [email protected]|
Third in a series on the role Berkeley Lab researchers are playing in planning for the proposed International Linear Collider.
Over a hundred groups of scientists from around the world are working together to design the International Linear Collider (ILC), an accelerator some 30 kilometers (19 miles) long that will collide electrons with their antiparticles, positrons, to create a realm of new physics – and the detectors needed to study it. Berkeley Lab physicists and engineers have a key part in this enterprise.
"The ILC will have the widest range of energy of any accelerator ever built," says Marco Battaglia of Berkeley Lab's Physics Division, who is also a professor of physics at the University of California at Berkeley. "It will be able to operate at energies from 90 GeV to 1 TeV" 90 billion electron volts up to a trillion electron volts "producing a large variety of different event topologies and possible signals of the new physics. This is a great opportunity for addressing many of the open questions in physics, but it poses significant challenges to the detectors."
To make some of the critical measurements for which the ILC is designed, its detectors will have to be more accurate by an order of magnitude in measuring the position and energy of the particles the machine produces, compared to detectors developed for other particle colliders which will require significant technological and engineering advances.
From the origin of mass to the nature of dark matter
Why will the ILC's detectors have to perform so much better than the highly sophisticated detectors already installed in today's accelerators? Consider the question of the origin of mass.
"In the Standard Model, the Higgs boson is what accounts for the mass of all force and matter particles," Battaglia says. "We expect that the Large Hadron Collider, presently under construction at CERN" CERN is the European Laboratory for Particle Physics near Geneva, Switzerland "will observe this last missing particle of the Standard Model. But to test that the Higgs boson indeed does it job of providing all these particles with mass, the ILC's ability to provide precision data is essential."
Higgs bosons will be created in the ILC when energetic electrons and positrons collide and annihilate. Higgs bosons will live a negligibly short time before decaying into force particles, such as pairs of W bosons, or into matter particles like the heavy quarks b (known as bottom, or beauty quarks) and c (charm quarks). In turn, quarks will rapidly decay into jets of lighter particles.
"We have to reconstruct all the particles coming from the decay of the quarks, the individual tracks of each particle from a b decay and a c decay," says Battaglia. "Finding a charm particle in the large background of beauty particle pairs – the most common decay mode of a light Higgs boson is a considerable challenge, which sets the parameters for the detector's performance."
The ILC will also search for an explanation of the puzzle of dark matter, which makes up a quarter of the density of the universe. Proposals include such inventive ideas as extra dimensions of space, but the most likely dark matter candidate, as predicted by several models of the new physics, is a new, weakly interacting massive particle, jokingly dubbed a WIMP. Because it should be possible to produce this particle in an accelerator laboratory, it should also be possible to study its mass and interaction properties in great detail.
The density of relic dark matter in the universe depends critically on many parameters, however. It will take a number of very accurate measurements to derive predictions from accelerator-lab data that can match the accuracy of cosmological data from more traditional sources like telescopes and satellites. As with studies of the origin of mass, detectors of high efficiency and great precision are essential to pin down the nature of dark matter and extract results comparable to satellite observations of the cosmos.
From new physics to novel sensors
Berkeley Lab physicists, engineers, and computational scientists have initiated an intensive program of physics theory, simulation studies, and sensor research and development aimed at building a bridge between the physics goals of the ILC to elucidate the great unanswered questions in particle physics and the performance specifications of each individual sensor in the detector, responsible for recording a single point along the trajectory of a discrete particle.
Berkeley Lab has a long tradition of developing advanced detectors and electronics for particle physics experiments, marrying scientific drivers with technical expertise through close collaborations among experts from different divisions.
Recent examples of contributions to tracking detectors include the CDF experiment at Fermilab's Tevatron; the BaBar detector at the Stanford Linear Accelerator Center; the Time Projection Chamber and the vertex detector upgrade being added to the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven; and the concept, design, and construction of the most advanced pixel detector so far, that for the ATLAS experiment at CERN's Large Hadron Collider.
The ILC detector poses even greater challenges. Outside the narrow vacuum pipe where the beams of electrons and positrons collide, it will have to measure particle trajectories with an accuracy on the order of one micron (a millionth of a meter). This requires a billion tiny pixel cells implanted in thin silicon wafers, each cell measuring about 10 by 10 microns for comparison, the ATLAS detector cells measure 50 by 500 microns each and each wafer only 50 microns thick 10 times thinner than ATLAS's.
"How can you read out all the information fast enough, before background events pile up and interfere with the accurate reconstruction of the particle trajectory?" Battaglia asks. "You can't do it the way you read a CCD, a charge-coupled device." (In a typical CCD the charges on all the pixels have to migrate to the edge of the chip before they can be counted.) "And because the wafer has to be thin, you can't build the sensor and its electronics on two independent wafers and bump-bond them together, as is done for ATLAS."
The alternative being pursued at Berkeley Lab, and at other research institutions, is to make the chip itself smarter. "So we are designing a device where the sensitive cell and its readout electronics are contained in the very same wafer," Battaglia says. Known as a monolithic pixel, this design is realized using commercial processing for CMOS chips (complementary metal oxide semiconductors), which ensures low production costs.
"We're tailoring our design to the performance of the ILC, but these developments are also significant in other fields of physics," says Battaglia. He points to sensors for electron microscopy under development in the Lab's Engineering Division, sufficiently similar to constitute what Battaglia calls "an important synergy of efforts," with Engineering carrying out chip design and the Physics Division's new Advanced Detector Lab performing the tests.
High-energy particle beams are no longer available on the Berkeley Lab campus, so tests of the new detectors make use of highly collimated beams from LEDs, or nanosecond laser pulses, which mimic the charge released in the silicon by an energetic particle.
Near the beam collision point, in the vertex region, monolithic pixel detectors will provide the high precision needed for tracking. But reconstructing the trajectories of charged particles over much longer distances thus making it possible to determine their momenta ten times better than hitherto achieved at CERN's Large Electron-Positron collider or the Stanford Linear Accelerator's SLD experiment will require a time projection chamber (TPC).
Since the time projection chamber was invented at Berkeley Lab by physicist David Nygren in 1974, many versions have been used in the world's accelerators. A TPC tracks and identifies particles as they fly outward through a gas-filled chamber, which is subject to a magnetic field that curves their paths according to their charge and momentum the greater the momentum, the flatter the curve. Along these paths, particles leave a trail of ionized gas molecules and electrons, which drift toward the ends of the chamber under the influence of a strong electric field. Detectors are positioned on these end caps, where they can record the arrival times and energy of ions and ionization electrons to determine the paths and energies of the particles that created them.
"The ILC will require a TPC with much better momentum resolution than other collider detectors," says Battaglia. "This means a large circumference and a strong magnetic field" engineering challenges that will benefit from Berkeley Lab's experience building the time projection chamber for the STAR experiment at RHIC "and will require improving the spatial resolution of the ions and ionization electrons arriving at the end caps. The idea being tested by the Berkeley group is to use microarrays of pads with highly integrated electronics, providing signal amplification and data reduction in a thin layer."
Such a design would represent a three-fold improvement over current designs while using only a tenth the material. The concept is similar to the pixel detectors designed for ATLAS, with their electronics bump-bonded directly to the back of the silicon, and existing ATLAS pixel chips make it possible to test the concept.
From sensors to experimental apparatus
Moving from the performance of individual sensor prototypes to the design of a final experiment that can reconstruct ILC events with high precision and integrity is a big step. At Berkeley Lab the effort to translate the ILC's physics program into detector specifications is well underway.
Berkeley Lab's detector design calls for a silicon vertex detector surrounding the beam pipe nested inside a time projection chamber. The design must be benchmarked against physics processes with well-defined requirements for accuracy, and it must be optimized in terms of both performance and cost. To build the infrastructure needed for physics simulations, detector simulations, and engineering design calls for years of effort and commitment, if the Lab is to play a leading role in the ILC's large-detector collaboration.
Not only Lab staff and UC Berkeley faculty members have made the commitment; UC Berkeley students also play a significant part. For two years, over 15 undergraduates have been engaged in ILC work under the supervision of Battaglia and James Siegrist, Berkeley Lab's Associate Laboratory Director for General Sciences and a professor of physics at UC Berkeley. What began as a discussion group now involves undergraduates in hands-on laboratory work to turn abstract concepts into the tools of advanced research.
Design, optimization, and research and development for the ILC detectors will continue over the next several years, as the overall ILC effort becomes increasingly focused. Hundreds of researchers from around the world are working toward the day which could come as early as 2017 when the ILC will deliver its first data, testing the fundamental properties of rare new particles as only a powerful and precise accelerator can, meanwhile forging essential links to studies of the cosmos.