Plans for the New Year

Just in time for Christmas, I received approval for posting a job ad (also on inspirehep jobs). I look forward to starting my own research at PNNL in earnest. So, to set the context, here's a bit of a rundown of what we're trying to do in the near future:
The ILC detectors are preparing to start TDR in the next year or two. For this we need to answer a couple of difficult questions convincingly. We'll have to justify the choices for all of the detector parameters. We haven't really changed them since the LOI in 2009. That was before the discovery of the Higgs boson. We'll need to take a good look at them again and study if they are still optimal and how much small changes affect the physics reach of the experiment. For this, we need to build new tools to better understand the connection between the detector parameters and the measurement precision. In a PFA detector, the reconstruction is a big part of the detector concept. I'm interested in studying the performance of the reconstruction under both physical and computational aspects.
In no particular order, here are a few things we already know that need work.

• Vertex reconstruction and flavor tagging:
• How does it perform with more background?
• Is the innermost layer at the right distance from the IP?
• What changes are needed to implement gaseous cooling?
• Silicon tracking
• What are the current limitations of the tracking, and can we overcome it with a pixel tracker?
• What can be gained from a different aspect ratio?
• Calorimetry
• How big of an advantage does digital calorimetry have over analog?
• What's the right size for the readout?

I'm sure there will be other questions that come up during the year. I look forward to working on these and other items with a motivated and capable scientist who will join us at PNNL. Let's roll up our sleeves.

FPCCD Neutron Damage test at Tohoku CYRIC

From afar the ILC detectors are going to have a very similar look to the LHC detectors ATLAS and CMS. They are cylindrical objects, with different sub-detectors for measuring different properties of the high-energy particles that will be produced by the collisions of bunches of electrons and positrons. Closest to the interaction point in the center of the detector will be the vertex detector for the precise measurement of the decay point of long-lived particles. Next, a tracking detector measures the momenta of charged particles from their trajectories in a magnetic field. The calorimeters outside of the tracking detectors measure the energy of the particles.
Yet, at second glance, because of much cleaner environment of the ILC detectors, particularly the inner detectors look very different from their LHC counterparts. While for the LHC detectors the requirement of radiation hardness puts a serious constraint on the achievable precision, ILC vertex detectors hope to achieve about an order of magnitude better precision on momentum and impact parameter measurements. Nevertheless, radiation damage is also a concern for ILC detectors. The beams after the collision cannot be re-used for collisions. (But we can hopefully recover at least some of their energy.) The current plan is to safely dispose of them in a beam dump. Some of the neutrons produced there can travel back up the beam pipe and enter the detector. Their energy has been studied and roughly looks like this.

 

Expected Neutron energies from the ILC beam dump.
The first layer of a vertex detector at the ILC has to deal with
up to 10¹⁰ neutrons / cm² / year

Not a big problem compared to the factor of about 1,000 that the LHC detectors have to cope with, but nevertheless something we need to study. We have a facility at Tohoku University where we can get a neutron beam. The energy is close enough to what we would expect at the ILC.

Distribution of Neutron energies in the
Tohoku CYRIC facility. We still have to
evaluate our own measurements of
 neutron flux and energy.

After some intense preparation and frantic manufacture of beam profile monitor from scintillator bars, we now have pixel detectors with the equivalent of about 2.5 years equivalent of ILC neutron dose. Analysis of the data is underway, so I can't show anything yet. We are still not even done with the complete analysis of last year's data. Here are two plots to give you just a rough idea of what we are studying. I will try to go into more detail in some future blog posts.

Active area of the FPCCD chip. The red dot is a "hot pixel".
Similarly to a dead pixel in your LCD screen, this is bad.
In this case, we define a "hot pixel" as one that has a signal
significantly above the (blue) background.

We are measuring the number of pixels that was affected
by the neutron radiation. As you can see, measurements
at low temperatures are significantly less affected by the
damage, but the difference before and after irradiation is
obvious.

LCWS14 Recap

Every year the Linear Collider community comes together for two large meetings. One is a regional meeting, usually held in spring. Every year this rotates between the three regions Asia, Europe and the Americas. The other meeting is the Linear Collider Workshop (LCWS), usually held in Fall. This is also hosted by one of the three regions in alternating fashion, and it's not the same region that holds the Spring meeting of the same year.
This year's event was in Belgrade.

View from the Fortress

The city still has not entirely recovered from the effects of the war over 15 years ago. As we were shuttled downtown for the summary sessions on the last day, we saw some derelict buildings with heavily damaged roofs at the outskirts of the city. Downtown, however, was very nice. There are plenty of options for dining, people were very friendly, and it was extremely easy to communicate in English.

The conference itself was held in two upscale hotels. Attendance was unfortunately not as good as previous workshops, but that did not stop the Serbian President from stopping by our little gathering. The sessions were a mix of plenaries and parallel group meetings. A notable item on the agenda was a session on how to make the ILC physics case. We were reminded on the first day that we should try to strengthen our presentation of the potential for discovery of new phenomena beyond the Standard Model. My colleagues and I believe we have a very strong case, so I guess we need to package it better. If you would like to contribute, post your video to the ILC communicators and let us know if you have suggestions how to present the ILC physics case.

In addition to politics we also did some real work. The reality of having a candidate site was very noticeable at the workshop. Both detectors have to agree on a common interface with the collider, which will lead to some changes in their design. And the design of the campus, including a computing center, is progressing as well. The work on estimating the computing requirements, both, for the proposed campus and off-site, unfortunately distracted me somewhat from carrying out my duties as a session convener. Thanks to my co-conveners, in particular Sophie Redford and Jerome Baudot, for picking up the slack.
The details of the computing will be the subject of a post once we have understood the details a bit better.

Goals for Detector Optimization: Mass resolution

We have several goals to optimize the ILC detectors to make the best use of the events that the machine will deliver. The physics case depends strongly on being able to deliver high precision and to cover as many new physics scenarios as possible that are still left after the LHC program. For this, we need to make sure the detectors meet our high performance standards. I have prepared an IPython Notebook that investigates the effect of the detector resolution on the reconstructed mass of gauge bosons. You can see that a seemingly small degradation in performance can have a big effect on the physics. We need to understand all of these subtleties when optimizing our detectors.