STFC-RAL-CR03 R61: Okay, thank you very much. It's really honored to be here today to talk to you about a new detector that we are building for the Atlas experiment for the upgrades, for high luminosity, Lhc. In particular, adding the possibility for ultra fast timing to help with the conditions that we're going to have during high luminosity.
STFC-RAL-CR03 R61: So maybe I just to give an overview and to set the stage a few introductory slides. Of course I'm sure everyone is already aware of some of these details, but just to set the stage. And why we want to motivate why we need to have a timing detector at Atlas for the Lhc, so I'm going to talk a bit about the challenges. Clhc. Put a bit of a picture of the upgrade program for for Atlas as well as the motivation performance of the of the Hgtv.
STFC-RAL-CR03 R61: this high granularity timing detector. And then I'm going to talk a bit about the technology choice that we made some of the R. And D of low gain avalanche detectors. This is a relatively new technology. In fact, it's come about in the last decade or so. So it went through an intensive R&D phase, including partly with the project for the radiation and some of the challenges and progress that we've had, and then finish with a few
STFC-RAL-CR03 R61: items to discuss the current status of the modules, the detector as a whole, how it stands, putting everything together, and how we're going to use it.
STFC-RAL-CR03 R61: So of course, I'm sure everyone's aware regarding the the Lhc. Which is a 27 kilometers circumference particle accelerator on the Swiss French border just outside of Geneva. Atlas experiments is one of the.
STFC-RAL-CR03 R61: It's 1 of the the 4 large experiments along the way
STFC-RAL-CR03 R61: which is located here.
STFC-RAL-CR03 R61: and the goal here is really proton-proton collisions largely to up to centers of mass energy, now of 13.6 TV, with the ultimate goal. To go to 14 TV with instantaneous luminosity is about 2 times 10 to the 34 cm squared per second, as well as proton bunch facing at 25 nanoseconds. This is quite vital because we're talking about collisions happening very fast and a lot of collisions happening around the same time.
STFC-RAL-CR03 R61: So when we talk about how the Lhc collides bunches of protons. So here we're colliding at the order of 10 to the 11 protons per bunch, and these bunches cross every 25 nanoseconds.
STFC-RAL-CR03 R61: So this results in in
STFC-RAL-CR03 R61: conditions where there are more than one interaction, every bunch crossing.
STFC-RAL-CR03 R61: And you can see here some of the the plots from Run 3 with the online luminosity. So this was the luminosity
STFC-RAL-CR03 R61: it was accumulated
STFC-RAL-CR03 R61: in 2023. You see, we we recorded over 30, inverse, actually delivered just over 30 recordings at 29.9 this year it was over a hundred inverse temptivar as well.
STFC-RAL-CR03 R61: and the conditions that we've had. When you look at the number of interactions per processing. So this is the mean number of interactions per processing. Every time the bunches of protons collide.
STFC-RAL-CR03 R61: You can see the steady increase in terms of
STFC-RAL-CR03 R61: these types of interactions year by year. So we're increasing the instantaneous luminosity. And that means more collisions, more particles to reconstruct, and more difficult environment for the detectors. And here you see, in 2022 it was 42.5 and 2023 51.2, and this year was up to 6. So this is really a very difficult and challenging environment in which to reconstruct particles from the collisions.
STFC-RAL-CR03 R61: Now, what does that mean? Moving forward? So these were the numbers that we had currently, during the 3rd run. So run 3 of the Lhc. And the next the next few years we'll have a an upgrade of the accelerator and of the detectors towards the high, low velocity Lhc. Which will be the flagship part of a physics project during the next decade. So the challenge here and the the excitement about high velocity Lhc. Is that we expect
STFC-RAL-CR03 R61: a integrated luminosity around or up to 4 inverse algorithm. And this is
STFC-RAL-CR03 R61: up to 10 times our current data set that we've had running out for for more than 14 years.
STFC-RAL-CR03 R61: So to do this, it requires an increase beyond what we have now in terms of instantaneous luminosities. And we're talking about a factor of 3 larger than we've had thus far, so up to 7.5 times 10 to 34.
STFC-RAL-CR03 R61: The result of this from the experimental side is the impact on the conditions that you'll see here you can see just a simulated event where you have mu equals 200, which is the expected pile up that you're going to have in terms of mean number of interactions
STFC-RAL-CR03 R61: with all of the different primary vertices and all of the different tracks that come. The charge tracks that are reconstructed from such an events. So this is quite a very dense environment for track reconstruction, and it requires a significant upgrade of the center. So that's at least setting the stage for
STFC-RAL-CR03 R61: how this, how this looks, and even to show this from a real event that was taken, I think, in 2017 this is said to be a new candidate event with 65 additional reconstructive vertices. So it's about a mu of 90, and you can see here, in the event, all of the vertices along the longitudinal direction. This is the direction which the primary vertices are reconstructed. So along the beam line.
STFC-RAL-CR03 R61: and all of the tracks which have a a track. Momentum transfers momentum of more than 100 mev. So you can see here
STFC-RAL-CR03 R61: the need for a very granular detector, with a very high precision to reconstruct so many tracks, and then to observe this set to new new events, you can see based on the the track selection how this screens, but we certainly don't just want to go to higher thresholds. Right? We want to to maintain the performance that we have
STFC-RAL-CR03 R61: with the current sector in such an increase in in conditions. The bottom left plot here shows as well the increase that we've had in terms of pile up across the various years across. Run one in Orange, Run, 2 in. Green, and run 3 in. Purple. You can see this general increase to get more and more accumulated. So this this, of course, adds
STFC-RAL-CR03 R61: quite a lot of additional challenges to both the online and offline performance of the detector itself.
STFC-RAL-CR03 R61: Why do we want this? I mean, I'm so far talking very negatively about the situation from the experimental point of view. But certainly that's something that we want as experimentalists. This very large increase in terms of the data set possible that with this high instantaneous luminosity, so to give you an idea, this means about 400 million Higgs Bosons will be produced at the end of high luminosity. So this gives us a very large data set in which to to make
STFC-RAL-CR03 R61: precision measurements and also to perform searches in new phase spaces. So it would take us decades longer running if we maintained the current instantaneous luminosity to accumulate such a data set.
STFC-RAL-CR03 R61: However, the the result is that we have to take into account changes. So the 1st is that the event sizes will be larger right. There are more collisions per bunch crossing many more tracks to reconstruct. So we have to accommodate for the larger event sizes. We need to to increase the granularity of the detector because of the higher detector occupancies that are very clear from all of the tracks that we see. So we're gonna have many more hits across the detector that need to be accounted for.
STFC-RAL-CR03 R61: We're gonna have higher trigger rates. So we need to redesign our trigger architecture and readout system, we have increased reconstruction, complexity.
STFC-RAL-CR03 R61: much more complex software is needed and as well. Another very critical aspect. And this is particularly the case in terms of the timing detector is the high radiation environment. So we need silicon that has tolerances to higher amounts of damage due to impacts of other radiation. So this requires the redesign of our current detector, or at least large parts of it, in preparation for the high luminosity.
STFC-RAL-CR03 R61: So just to briefly flash, this is a photo of the Atlas detector, of course, with the inner detector. So the silicon in the innermost parts, followed by the calorimeters, and then the neon system with the toroid magnets. As I mentioned, we don't need a substantial upgrade. Of course it doesn't do it justice just to put a few points.
STFC-RAL-CR03 R61: but, as I mentioned. We'll need upgrades for the higher rates in the trigger and data acquisition. All of the electronics need to be upgraded from the back end of every single system to handle the readout. We'll have an additional muon coverage to cover some of the holes we have currently.
STFC-RAL-CR03 R61: then, as well. The big upgrade is the new, fully silicon tracker which we call the itk, and this will also have an increased coverage in suitability. So it'll have more coverage beyond suitability of 2.5 up to suitability of 4, and I'll show you what that means in a slide or 2,
STFC-RAL-CR03 R61: and as well.
STFC-RAL-CR03 R61: The in inclusion of this novel Http, which is a silicon timing detector, which is primarily focused in the very forward regions. And I'll show you this in a second. So, looking at the forward regions of the detector close to the the beam lines where Pilot will be quite well, severely impacts the performance.
STFC-RAL-CR03 R61: I I really liked this slide just to set the stage of why this is such a big upgrade, and why this takes so long to to build the detector and then to install it so to give you a feeling. The current Atlas.
STFC-RAL-CR03 R61: the Sct. So the silicon strip detector and the current Atlas Pixel detector are shown in the shaded regions.
STFC-RAL-CR03 R61: These lines in blue and red are the Itk detector. So the upgraded sector. You can just see the sheer size of it compared to our current detector. So this just shows the level of complexity. 10 times more number of channels. We're talking in a giga. So billions of channels to read out, and as well. The granularity is increased significantly. You can see just the by the color shaving on the left.
STFC-RAL-CR03 R61: the current pixel size current ibl size, which is the innermost layer, and then the
STFC-RAL-CR03 R61: pixel pitch
STFC-RAL-CR03 R61: for the innermost layer, and the rest of the layers, which is drastically increased. You can just see it from from the shaving colors.
STFC-RAL-CR03 R61: So this is a very, very large upgrade, and when we talk about how this looks in terms of the coverage, the current detector that, you see, goes roughly up to a suitability of 2.5, which is about here.
STFC-RAL-CR03 R61: and the future inner tracker will go up to a pseudo 80 of 4 in terms of average. You see, it's a very small solid angle into the forward region
STFC-RAL-CR03 R61: when looking at how the performance will be impacted by the pile up. So the increase in collisions looking at, for example, the pilot density. So this is the number of vertices on the left hand side per millimeter that we'll have for mu equals 30 and mu equals 200, and you can see the very large increase. In fact, the median value of this curve is around 1.6,
STFC-RAL-CR03 R61: and that means that we'll have a lot of vertices per per millimeter, and if the
STFC-RAL-CR03 R61: resolution of the tracking in the in the longitudinal direction is degraded, which it is in the forward region you see on the right hand side here. So this is the resolution in the longitudinal direction for tracks of one Gv. And 10 gev. And what you see, of course.
STFC-RAL-CR03 R61: just natural. We're looking at smaller and smaller solid angles. And thus the resolution back in a given direction is degraded.
STFC-RAL-CR03 R61: becomes a point where you can no longer associate a unique track to a unique vertex, and then you have tracks which could be associated to multiple vertices. And when you're looking for this, this single vertex which has the hard scatter event.
STFC-RAL-CR03 R61: this becomes quite a bit of a challenge with all of this additional pilot, and with the resolution of the tracks in this case. So one of the one of the goals here is to improve the performance in this longitudinal direction, for track to Vertex Association in particular, when going to higher and higher suit abilities.
STFC-RAL-CR03 R61: And how do we do this? We use the fact that the beam spot not only has longitudinal spread of the interaction, but also has a temporal spread. The temporal spread is about 180 people seconds in the current design. So if we're able to measure a time to track, we could improve the track to Vertex Association for tracks which are in this region.
STFC-RAL-CR03 R61: So, as I mentioned, the vertices have a spread of 180 picoseconds. So this is a simulated event where you have the hard scatter interaction which we're interested in place with all of the overlapped interactions. So these are all pileup interactions.
STFC-RAL-CR03 R61: And if we're just using the longitudinal direction, you can see within the resolution here that you'll have many vertices which are overlapping in terms of just this longitudinal direction.
STFC-RAL-CR03 R61: When we use the spread of the time, we can disentangle them across from each other. So this really gives us a handle, then, to reduce the tracks
STFC-RAL-CR03 R61: from pilot events, whether they're from real pilot interactions or whether there's capacity tracks. So tracks that are just from hits because of the increase in number of hits reconstruct a track to, for example, adjust this reconstruction information, and we can reduce all of this with the the time that's associated to tracks.
STFC-RAL-CR03 R61: Now going a bit into detail of the Httv. Itself. So, as I mentioned, it's going to be silicon coverage between the pseudopinity of 2.4 and 4 in both ncap regions. It's going to be located about 3 and a half meters away from the interaction point. This is right outside of the inner tracker volume. The inner tracker volume, roughly, is 2 meters by 6 meters. So we're going to sit just beyond that in both directions, right onto the liquid argon and cap biostat right in front of the caliber.
STFC-RAL-CR03 R61: The technology of choice here is located in avalanche detectors to aim for a timing resolution per track of 30 to 50 picoseconds. The idea behind this is that with 180 picosecond spot spread, this will allow us to reduce the the impacts of of pilot tracks
STFC-RAL-CR03 R61: which are associated to our scanner. And here the performance is is quite optimal
STFC-RAL-CR03 R61: and as well that one of the very key aspects of this. And I'll go into a bit more detail because this was the key factor in terms of our study of this technology is the impact of radiation damage. So in this forward region, you can also imagine there's a very large number of particles, large flops of particles that the detector will see in particular when looking at the very low radial regions of the detector. This is
STFC-RAL-CR03 R61: a radius about 10 cm from from the B model.
STFC-RAL-CR03 R61: and so very, very close.
STFC-RAL-CR03 R61: and here, of course, the occupancy will be very high. The flux of particles will be very high, and this will see a significant amount of radiation.
STFC-RAL-CR03 R61: And so the the goal here was to really study and understand. Lg. Performance after very high fluids. In this case, 2.5 times 10 to 15.
STFC-RAL-CR03 R61: The sensors also will be operated at minus 30°C. Again, to reduce impacts that we see because of radiation. And here we're using Co 2 dual phase cooling. So this mixed state between gas and liquid that drops the temperature because of pressure.
STFC-RAL-CR03 R61: And you here on the diagram, the different shadings are different lgap modules. I'll mention on the next slide of the reason why we have different shadings. So the active area goes from about 110 to about 660, depending on the exact module location on the outside are the peripheral Electronics board. So this is where we convert the signal, for example, from electrical to optical that sent them to the counting room. Also, all of the Lpgs are located, and this
STFC-RAL-CR03 R61: location is chosen because it's the furthest away possible, so sees the least amount of radiation.
STFC-RAL-CR03 R61: Another main challenge of the detector is to optimize within the constraints that we have. So this location, of course, is very constrained in spatially as well. So we are talking.
STFC-RAL-CR03 R61: As I mentioned, then, the the radial extent of this vector for the active area is about 660
STFC-RAL-CR03 R61: The center itself extends just beyond that. So about a radius of just over a meter.
STFC-RAL-CR03 R61: The reason for this is that we are very spatially when Atlas is closed, very, very spatially constrained also in the radial direction where the services are routed out.
STFC-RAL-CR03 R61: So what's optimized in this case is that we want the the largest number of hits per track. So we want more than one hit per track
STFC-RAL-CR03 R61: because the time resolution goes as a function, you know, square with N. If the time resolution is independent between individual hits. So we optimize to have
STFC-RAL-CR03 R61: layers of silicon on 2 discs on both sides. So up to 4 hits, you see the 2 disks, the figure on the left, where you can see at least the front side. But there's also a backside to each case where where the located this all has to fit within 12.5 cm, and that includes the inner and outer moderator parts. So it's extremely tight. This is another really difficult aspect.
STFC-RAL-CR03 R61: I'm already that boy.
STFC-RAL-CR03 R61: Yeah. Okay, great.
STFC-RAL-CR03 R61: Only monthly. 6 month price. Yeah.
STFC-RAL-CR03 R61: And so, as I was saying, yes, so it's very tightly packed location. So there was a lot of optimization in terms of the detector itself how to optimize, and the overlap. So the coverage of silicon per track.
STFC-RAL-CR03 R61: So in the innermost parts, there's about a 70% sensor overlap between front and back sides in the middle part about 50, and the other part is about 20. Those are the different shavings, you see. That's also because the occupancy is much higher in the innermost parts, and the radiation damage will cause a degradation of timing performance in the innermost part. First, st so the more hits we have, the better the timing performance over time.
STFC-RAL-CR03 R61: And here you can see how the hits can be located on on one side of the detector. On average, we have about 2 hits, and we also rotate the the 2 disks with respect to one another, to maximize the the coverage and to minimize more than maximize, we minimize the dead regions. Because there's a lot of dead area in this segment.
STFC-RAL-CR03 R61: Then, just quickly, briefly highlighting how this looks. Http and lance that we're using are hybrid detectors. So this is a sensor which is bump bonded to a front end asic
STFC-RAL-CR03 R61: per module. We have 2 elgat sensors and 2 front end asics, which are then glued to a module flex.
STFC-RAL-CR03 R61: So each of the sensor and ultra asics are symmetric, 2 by 2 cm squared, and then they're they're combined together into one module, which is 2 by 4 cm, where you see here on the on the bottom side.
STFC-RAL-CR03 R61: And yeah, so there's a lot of challenge in terms of the hybridization. These are these bumps are quite big. Our pad size here that we optimize to is about 1 by 1 squared. So the bumps are relatively large, but they're spread apart quite, quite a lot quite a lot more than, say, your typical pixel detector with tens of microns of pad size.
STFC-RAL-CR03 R61: So they're spread quite a lot apart. So this is a challenge for the hybridization.
STFC-RAL-CR03 R61: We then glue the module flex and then wire bonds to the front end asic and the sensor. The other aspect of timing detectors which we can talk about later if you want to, is the fact that timing detectors consume a significant amount of power. This is the other big limitation of timing detectors is the amount of power that they consume, and that's
STFC-RAL-CR03 R61: another very important aspect of the testing that's ongoing now, and the checks that we're doing are the fact that there has to be very good thermal contact between the cooling plates where the Co. 2 is, and the sector itself in particular, the A/C. Which consumes the majority of the power.
STFC-RAL-CR03 R61: And then this module flex is just a standard PCB. Which is wire bonded and attached to a flex scale, and the flex scales run out
STFC-RAL-CR03 R61: along the radio direction through the virtual electrons, and I'll show a photo. Actually, the detector we have now built prototype detector as part of the demonstration of this technology. So when we talk about module, this is the elements we're talking about. And this is combined into a peak
STFC-RAL-CR03 R61: produced support units which are then screwed to the cooling plate to form such a path.
STFC-RAL-CR03 R61: Okay, it's a bit of a you know.
STFC-RAL-CR03 R61: square in a circle. So that's where you start to see some of the dead area and some of the optimizations. In terms of this, this detector itself, and each quadrant is then replicated 4 times, and then front and back, and then
STFC-RAL-CR03 R61: so Lgat. So, as I mentioned, this is a relatively new type of technology. You can find literature from it. The order of 10 years ago this started to come about. This was developed in collaboration with Cnn. Which is a foundry in Barcelona, and the Rd. 50 collaboration at Cern. And the idea is that it's a standard segmented, and P. Silicon. So you have your PIN diodes.
STFC-RAL-CR03 R61: and there's a very highly doped layer which we call either the gain layer or the multiplication layer, which creates a very large electric field right at the junction.
STFC-RAL-CR03 R61: So this is where the avalanche comes in, and this increases the signal noise to noise, which is proportional to your time resolution measurement in terms of the jigger.
STFC-RAL-CR03 R61: So the very high electric field causes this avalanche region, the gain that we're targeting when we talked about. It's low gain, but let's say, moderate gain of about 10 to 20. This is where the the most optimal performance is for signal over noise
STFC-RAL-CR03 R61: because of the multiplication layer, it also allows us to go thinner. Another aspect of timing layers is that when we talk about the time, resolution, we also not, are not only interested in the signal over noise. We're also interested in the rise time of the signal itself.
STFC-RAL-CR03 R61: and this is limited by the the active thickness of the silicon. Right? So thinner sensors have very fast rise time. And that's better time.
STFC-RAL-CR03 R61: Okay, so we optimized it down to 50, 35 microns. We originally started with about 250 microns of active area, so thinner detectors also better for material and better for time resolution. So here already very early prototypes showed very good time, resolution, less than 30 ms, pre-irradiation.
STFC-RAL-CR03 R61: 2 words on the front end asic. So this is really the very challenging aspect of the project as well as you can imagine. This front end Asic has to withstand very high levels of radiation in terms of total ionizing dose.
STFC-RAL-CR03 R61: So we are talking about 2 mega grid.
STFC-RAL-CR03 R61: and as well because of the spatial constraints and the granularity of the detector. We have to be able to read out every single channel, and that means that there have to be. There has to be a readout for every channel in the Asic itself. So it's very, very spatially a difficult problem.
STFC-RAL-CR03 R61: So we have a preamplifier discriminator to capture the Elgat signal and then for timing. We're not just interested when the particle hits, and I'll explain why in a few minutes. But we're also interested in the sort of the total collected charge for a given hits. So we need not only the time of arrival, but we need also the time that the signal spends over a given threshold.
STFC-RAL-CR03 R61: It would be great to just capture the whole waveform and read it out, and we had. I don't know, 6 million oscilloscopes sitting in the counting room. It would be perfect, but of course we have to digitize this information on ship, and this is very spatially a difficult problem. So we use 3 new delay lines for this for both the time of arrival, the toe and the time over threshold the tot.
STFC-RAL-CR03 R61: Then also we move to the digital part of the Asic, which then has to to store everything in local registers and within buffer, which is a byfold buffer. So 1st in 1st out, so it's stored over a certain amount of time waiting for the Atlas signal to accept the event. So the latency here we have to hold on the detectors. 35. So that's another very challenging aspect of the memory.
STFC-RAL-CR03 R61: The other very important aspect of of Http is that it will also provide in a dedicated readout path luminosity. So Kit, counting
STFC-RAL-CR03 R61: the reason that this is quite important is that certainly luminosity is one of the limiting factors in terms of precision measurements at the Lhd. So HD. Will also provide luminosity at 40, Megahertz. So every 25 nanosecond bunch facing out
STFC-RAL-CR03 R61: there are very strict jitter requirements for this detector because of the impact of irradiation on Ldads, and they are that we need less than 25 picoseconds for 10 femtocoulomb. 10 femtocoulombs is about a minimum ims in particle.
STFC-RAL-CR03 R61: And that's this diminishes with the radiation down to 4 Pentacoulomb. And we need to maintain 65 seconds in terms of the electronic shooter. So this is also a very, very difficult problem in particular, when it comes to the lower thresholds. Here we're setting discriminator thresholds at the order of 2 Pentacoulombs. So 1 5th of a minute. This is a very big range, and very difficult in order to to meet the performance down to 4 pentacolumn.
STFC-RAL-CR03 R61: How does the Vernier delay line work? So we have a configuration in 2 delay lines, a stop and a start.
STFC-RAL-CR03 R61: This is where a lot of the power consumption comes from in time of detectors because you're running. Then, when you have an event detected, you start to run a clock right, which is flipping every 120 people seconds, and the more flips you have, the more bits you keep.
STFC-RAL-CR03 R61: the better the time resolution is. So you want to go as low as as many as possible to get the best high resolution.
STFC-RAL-CR03 R61: And so what happens is that when you have an event detection, the clock starts, and this is a slow line with 140 picosecond flips. This all has to be controlled in asic. And then at the end of the measurement window. In this case, about 2.5 nanoseconds around the optimal location of the potential particles. From that interaction
STFC-RAL-CR03 R61: the fast line starts, and when this catches up you get your time of right number right, and the bin granularity gives you that time resolution, uncertainty, which is about 20 gigaseconds.
STFC-RAL-CR03 R61: And, as I mentioned, this is really the the largest consumer of power. Here we're talking about 300 milliwatts per centimeter squared. So even though we're a small detector
STFC-RAL-CR03 R61: compared to the inner tracker, we still require a substantial amount of Co. 2 cooling to maintain the the sector performance, even at high irrigation.
STFC-RAL-CR03 R61: Okay.
STFC-RAL-CR03 R61: the other important thing, as I mentioned, is luminosity. So occupancy can be, at least in certain cases, linearly correlated with the number of interactions. That's the key point of Http compared to other luminometers which are saturated between low mu and high mu. So one of the key aspects is that we're also summing the hits in a given window
STFC-RAL-CR03 R61: at each bunch crossing in order to send out at 40, Megahertz, regardless of a level 0, except from the atmosphere. So this is quite important for luminosity, determination. Here you can see from simulation how this looks in one of the layers. We only use the largest radial parts of the detector. To read out luminosity would be too difficult in the innermost part wouldn't have very good occupancy correlation.
STFC-RAL-CR03 R61: But you see here the linear correlation
STFC-RAL-CR03 R61: for for different windows here in this sector, in the 1st layer, which is very good all the way up to
STFC-RAL-CR03 R61: you got around 200. This is very important for luminosity, and it's why Httv will be one of the luminometers for atlas for the high luminosity of HD.
STFC-RAL-CR03 R61: So, as I mentioned, time, resolution involves several different components.
STFC-RAL-CR03 R61: so we usually write it out like this to to talk about the different contributions. The important thing to mention here, too, is that
STFC-RAL-CR03 R61: while most of the time we talk about the silicon right, the algorithm. In this case there's a very complicated relationship also with the front end asic. And if there's something just to remember is that aspect, too. So it's a minimization of the total system. And I'll explain to you why that is so when we talk about the landau fluctuations, we're talking about ionization, non-uniformities in deposition silicon, so standard
STFC-RAL-CR03 R61: fact that you have non-uniformity. So you have just differences. Even if you have the same particle that crosses the silicon here, you can see that this is the variation in terms of the shape of the Algon signal and the amplitude variation that we see. So this we can minimize by going to thinner sensors. And here we can tell from our 50 microns that this is less than 25 picoseconds in thin sensors.
STFC-RAL-CR03 R61: The next contribution, which is the large contribution. When people talk about time resolution, they're usually talking about the jitter contribution, and, as I mentioned, it, goes roughly as the rise time divided by the signal over noise, so quicker. Rise, time gives you lower, jitter higher, signal over noise gives you lower jitter, jitter, you can imagine, is as much of this right when you cross the threshold. If you have a very long signal that takes a long time with a lot of noise, you get large variations when it would cross fixed threshold.
STFC-RAL-CR03 R61: That's how one can imagine it. The other is the time, walk, contribution. So when particles of different energy or different charge different energy. Sorry. Cross into the detector you have differences in when a fixed threshold is met, even if they came at the same time.
STFC-RAL-CR03 R61: Right? So you have 2 particles that arrive from the detector at the same time, one that deposits a little bit of energy, one that deposits a lot of energy, and you have a difference. And this is what we call the time off.
STFC-RAL-CR03 R61: And this is the reason why we have a time over threshold.
STFC-RAL-CR03 R61: Tdc. As well, because this can be corrected by Cap by also
STFC-RAL-CR03 R61: calculating the time if the signal is above the given fixed threshold. Okay, so this is the reason in timing and precise timing. We also need the second
STFC-RAL-CR03 R61: at T. Ot. To be digitized and and sort
STFC-RAL-CR03 R61: to go through this. So the idea is that the largest components from the jitter jitter. We target pre irradiation about 25 picoseconds, and that went into all of the design of the detectors itself.
STFC-RAL-CR03 R61: Then the the other part, which is also quite quite important, is the Tbc granularity. There's also some fluctuations
STFC-RAL-CR03 R61: bin by bin in terms of the Tdc population which have to be calibrated
STFC-RAL-CR03 R61: right in each front end. Asic. This has to be done. There are also the variations due to the fact that the granularity could be larger. One could design a granularity which isn't 20 picosecond per bin, the other are the clock distributions. These are another important aspect regarding the overall central clock of the Lhg, which then needs to be sent to Atlas, and needs to be distributed from the central trigger processor. So from Lhg to Atlas, then distributed to all of the front end asics
STFC-RAL-CR03 R61: right? And so each of these, so called, I guess, hops you will, if you will, between transitor and receiver, also need to be understood and need to be within a very small jitter need to be need to have very good time resolution as well to distribute the clock down to each module.
STFC-RAL-CR03 R61: So, as I mentioned, there's not just the silicon aspect. There's also the hybrid aspect. So, as I said, Htm, modules are hybrid silicon detectors. So it's an L gap bombarded to an Asic. But the electronics jitter comes from the preamplifier which goes roughly as the capacitance of the detector divided by the injected charge.
STFC-RAL-CR03 R61: As you can imagine, the capacitance is correlated to the thickness, the active thickness. So while you want to go thin as possible for the silicon. You want to go as big as possible for the Asic. So you have these 2 contributions which are competing against one another. So this is why we settled on 50 micron active area, which corresponds to a capacitance about 4 people. Fahren. Okay, so this was the. This was the optimal
STFC-RAL-CR03 R61: optimal choice for us in Http, based on, as I mentioned, the rise time signal, the lambda fluctuations. Certainly, of course, going to thinner. You have less power. You have less material, so less bias, voltages which are applied, less leakage current
STFC-RAL-CR03 R61: and as well. There was a lot of optimization in terms of the multiplication layer. So I said nothing so far about what this multiplication layer is. But there's a lot of limitations and different types of studies that were done in terms of the dopant materials, the depth, the diffusion profile of this material inside the right, and looking at different dopants. This was part of the R. And D program of these Lgas until today.
STFC-RAL-CR03 R61: So that I said a bit about the occupancy. So this is what it looks like in terms of radiation, damage and influence in neutron equivalents, so still again led for neutron, for neutron equivalents.
STFC-RAL-CR03 R61: You can see here the total which gives us around 6 times 10 to the 15 neutron equivalent, which is too high for our cats. So what we're going to do is we're going to replace
STFC-RAL-CR03 R61: the detector increased 3 times. So 3 times in the innermost part and once in the middle to reduce the impact. So there's another limitation because of these issues with the radiation which I haven't explained to you yet, as well as the mineralization due to mitigation through the Co. 2.
STFC-RAL-CR03 R61: So there's another complicated aspect of the detector needs replacement of the hybrids and modules actually overall modules several times during lifetime to reduce the irradiation.
STFC-RAL-CR03 R61: both in terms of fluence and the dose.
STFC-RAL-CR03 R61: The the other reason that this is a very complicated problem.
STFC-RAL-CR03 R61: Is that where we are in the Ford region of Atlas, here you can see the inner tracker. But it looks like you have hydronic interactions in air. If this isn't air, there's a lot of material in front of the detector. So the other complicated aspect, not only in terms of reconstruction of tracks is hydronic interactions which also increase the radiation problems. So we have much higher number of particles which are impacting the detector in this region due to the services from the inner tracker. So there are a lot of complications in this region as well
STFC-RAL-CR03 R61: in terms of detector performance.
STFC-RAL-CR03 R61: So let me go through. Then briefly, over the course of a few slides, the Lgad radiation study. So this is the really the core R&D, that was done within the community and also within Http. To come to the point, we are today.
STFC-RAL-CR03 R61: So starting off with very early tests, so these are from 2,017 of tests that were done in the lab on Cnm. Sensors at different values of irradiation so non irradiated, which are the original one, and up to 2 times 10 to the 15.
STFC-RAL-CR03 R61: So what we typically do is we irradiate neutrons at the Igs lab, or at least the the nuclear facility in Ljubljana, outside, just outside of Ljubljana, as well as protons at the Iraq facility at Cern.
STFC-RAL-CR03 R61: What you'll notice the trends here, and the important thing to note is that with the radiation damage, you'll notice that the most probable charge. So this is the collective charge in a sensor decreases drastically with the radiation. So that's the 1st trend you start to see from here, where you run at a bias voltage about 100 volts
STFC-RAL-CR03 R61: all the way up to 2 times 10 to the 15. And here you have gain. This is your gain can be can be calculated from the most probable charge which is drastically reduced.
STFC-RAL-CR03 R61: Okay, so you lose the game as you irradiate the sensors.
STFC-RAL-CR03 R61: The results is, of course, then, as well for the same thresholds that are set
STFC-RAL-CR03 R61: in the in the collected charge or cut on the on the collective charge, you lose efficiency.
STFC-RAL-CR03 R61: So eventually, at some point, that's why we reduce the the the requirements on the cuts in collected charge
STFC-RAL-CR03 R61: throughout the lifetime of inspector
STFC-RAL-CR03 R61: to to maintain the the full efficiency of the of the detector. And here we're not talking a very high cut. Right? We said, 1010. You're already down to 75% sufficient
STFC-RAL-CR03 R61: one of the ways to maintain
STFC-RAL-CR03 R61: the performance. And you notice that immediately from this plot, so not only does it degrade, so if you were to stay at the same working point in terms of detector high voltage. You would just go down the vertical axis, and you go from a gain of about 20 down to a gain of of one or less
STFC-RAL-CR03 R61: for the same bias voltage. So the other aspect is to increase the bias voltage in order to maintain the performance throughout the lifetime.
STFC-RAL-CR03 R61: Here you go from about a hundred volts
STFC-RAL-CR03 R61: to 700 volts for this sensor type, which was a very early prototype from Cnn.
STFC-RAL-CR03 R61: There were also a lot of studies in terms of understanding different types of dopants in the multiplication layer. So the original dopant that we were using was boron. You have different types of diffusion patterns, different concentrations at different depths.
STFC-RAL-CR03 R61: But overall, what you what you can see from this this plot on the left hand side is that for many other, at least all of them
STFC-RAL-CR03 R61: at a certain point, which is above a fluency one times 10 to the 15.th So not even at the maximum fluence with Cn. Hdt. You see the degradation of the game layer.
STFC-RAL-CR03 R61: So there's a loss of the gain layer due to irradiation for all of the different types of dopants, and this is now understood to come from the acceptor removal process. So you have a loss of the Boron concentration. For example, Boron. Here was the choice, the original choice of dopants in Http. So we studied all these different types
STFC-RAL-CR03 R61: in lab and in testing to try and maximize, then the the performance and the the game layer loss.
STFC-RAL-CR03 R61: These are some of the the figures that we did and studies that we did in the lab to check the impact of the what we call the the gain layer, depletion, voltage. Right? So what is the voltage at which your gain layer is fully depleted, which changes as a function of fluence. So on the horizontal axis you have the fluence detector, and different sensors.
STFC-RAL-CR03 R61: So different sensor runs from different countries, different vendors which have different properties of the the game layer. And what's very clear, even without I'm not through all the sensor text, but you can really see by eye that there are 2
STFC-RAL-CR03 R61: slopes in terms of the exponential decay, one which falls very rapidly, and one which stays very steady, and this you can then plot as an exponential falling curve and plotting the constant. You see that there's a trend where carbon enriched
STFC-RAL-CR03 R61: wafer production increases. Then the the performance of the game layer up to higher frequency
STFC-RAL-CR03 R61: roughly, is the plot before, but now shown for all of the sensor types for Http. So here carbon enrich wafers, achieve the lowest acceptor removal overall, they maintain their performance for the longest time.
STFC-RAL-CR03 R61: Here are some photos of some of the extensive R. And D, so most of the runs, because Cms is also building an end cap, timing layer that on their program, which is very, very similar.
STFC-RAL-CR03 R61: also using lcats and similar original front end asic design.
STFC-RAL-CR03 R61: And you see some of the runs that we had in the shared R. And D. Wafer runs from different fundrais, different vendors in Japan, Hamatsu, Fbk in Italy, Cnn. In Spain, and the Chinese runs that we did, too, at Ime with different sensor designs, one from Icap in Beijing, and one from Usdc. In china.
STFC-RAL-CR03 R61: That was the photo you saw
STFC-RAL-CR03 R61: from the same round this 8 8 inch wafer. Run that line.
STFC-RAL-CR03 R61: and yes, to to plot. Then how these different gain layers also look, as I mentioned, in terms of efficiency. You can see the loss of efficiency for Boron in particular, which was efficiency, same with Gallium, which we had hoped would be more radiation. Hard? Of course not.
STFC-RAL-CR03 R61: Carbon is the best performance similar on the bottom, for the most probable value of the charge. This follows the lambda fluctuation roughly. So we're talking at the peak of the distribution.
STFC-RAL-CR03 R61: where, when you go across in order to maintain 4 centric, which is roughly the target, as I mentioned earlier, for collective charge. If you have a Boron sensor or gallium sensor, you're forced to go to 700 volts in order to to achieve this performance.
STFC-RAL-CR03 R61: Similarly on the right hand side here these were Hpk runs with different types of adobes. Hpk. Did not want to add carbon. They actually preferred to go a different route, but overall the same performance without carbon. You're forced to increase the bias voltage up to 700 volts in order to maintain
STFC-RAL-CR03 R61: the target time, resolution, and target collective charge.
STFC-RAL-CR03 R61: I want to talk about something that's happened over the course of several years, which was a big mystery. So, putting this all together that I showed you.
STFC-RAL-CR03 R61: we had many testing campaigns between Atlas Cms and Rd, 50 and all different types of Lg testing. And throughout the years and even past 2019, what we noticed were losses in terms of sensors due to some
STFC-RAL-CR03 R61: something some factor was causing that when we were above a fluence of 1.5 times 10 to the 15, we would have fatalities in the sensors. Total loss, and you can see under a microscope how this looks. This is one from a 2018 test beam. It's 1 from a 2020. These are physical burn marks. There are craters in the sensor.
STFC-RAL-CR03 R61: and there was a big campaign to try and understand this. These incidents were never observed in the lab, so no infrared laser resulting in such a in fact, no data source testing resulting in such an impact. But this seemed to happen
STFC-RAL-CR03 R61: somewhat at random, of course not random, but the the goal was to try and understand this.
STFC-RAL-CR03 R61: And so now you can find this quite detailed in literature. The 1st thing we did was to try and understand if this was due to 2 particles at the same time, whether it's due to environmental conditions, we were able to trace back, reconstructing the event and capturing the waveform, so the actual signal of the elgan when it died, and the resulting particle track.
STFC-RAL-CR03 R61: And so we were able to conclude this was from a single particle event.
STFC-RAL-CR03 R61: And the working hypothesis that now is quite extensively used to to understand this process is this single event, burnout. It's now called.
STFC-RAL-CR03 R61: where you have a highly ionizing event which causes a short. So you have high voltage on the backside, and your electric field is much larger than the critical field for that thickness.
STFC-RAL-CR03 R61: and this causes a shortened discharge in the sensor.
STFC-RAL-CR03 R61: So what we did then is to try and parameterize when this happens, and so joint collaboration between Atlas Cns and Rd. 50 to try and understand where this issue occurs, and if there are safe zones to operate. And there were 2 things that became very clear after looking at over 50 sensors over the course of several years was that it's correlated with the thickness of the sensor
STFC-RAL-CR03 R61: right? And it's correlated with the bias voltage that's applied.
STFC-RAL-CR03 R61: And what we found is that when looking at different thicknesses, so going from 35, 45, 50, 55, we're able to set a very nice straight line where we had our 1st incidence of this occurring, and so we're able to determine a safe zone to operate these sensors, which is less than 11 volts per micron. So a 50 micron sensor cannot operate above 550, and I think by now you understand where I'm going with this.
STFC-RAL-CR03 R61: So unfortunately, this is an additional constraint, constraint for Http. And everything I showed you showed you that we needed to go to 700 volts in order to operate this detector. Okay? So this was a, this was a big effort and a big unknown. Why this is happening. So that's the understanding we have today of this process. So this is also within the past couple of years.
STFC-RAL-CR03 R61: So looking at, then, how this looks in the lab, because in the lab we can run the sensor safely beyond the the highest voltage of 550 volts, and you notice immediately, when looking at
STFC-RAL-CR03 R61: the most probable charge and the time resolution, that there's a trend with carbon and without carbon, so without carbon, no way, you're well within the area which is prone to single event, burnout, and that's
STFC-RAL-CR03 R61: cause irreversible breakdown in the sensors
STFC-RAL-CR03 R61: with carbon implanted sensors in the lab. We could meet the criteria
STFC-RAL-CR03 R61: where you see on the left the most probable charge.
STFC-RAL-CR03 R61: As these are the highest fluences, where you can run successfully about 4 of them
STFC-RAL-CR03 R61: and maintain then the- the performance below 550 volts, same for time resolution. As I said, we have multiple hits. So the target is 70 picoseconds per hit.
STFC-RAL-CR03 R61: and here we're down to 40 picoseconds with carbon enriched sensors.
STFC-RAL-CR03 R61: And finally, we did a testing campaign in 2021 and 2022 to prove this, at the Sps. And the Daisy, where we have 3 different wafers, which are carbon invented.
STFC-RAL-CR03 R61: one from, and 2 from Ime, and you see that we meet all of the criteria within within these additional constraints, so below bias voltage of 550 volts.
STFC-RAL-CR03 R61: See that we meet meets the requirements of collected charge, which is 4 and a time resolution, which is below 7.
STFC-RAL-CR03 R61: Okay, so this was really, this was a killer of the project at the time, because we had no idea where this was coming from, and nothing fit within the the constraints of the inspector itself. So carbon implanted is the default
STFC-RAL-CR03 R61: for Atlas, also in the high irrigation zones for Cms as well. After these studies, and in Http we'll use all of our sensors from China.
STFC-RAL-CR03 R61: Actually the the vendor that won the-them temporary Is- is the Chinese vendor that I am. So all of the sensors will be produced in China.
STFC-RAL-CR03 R61: Then in 2 min, just to leave you with a few plus and and points on where we are today.
STFC-RAL-CR03 R61: so that that I gave you a lot of information on the hybrids elements themselves. So the Lgat, R&D, the the front end Asics. But now we're building. Now, hybrid supports are being tested. The final prototype has already been tested extensively. In lab testing. You can see some of the loading elements. So the these are our modules, which are then glued to the support unit on a vacuum plate. This is done at 6 institutes in Htt.
STFC-RAL-CR03 R61: And then it's shipped to Cern, where it's assembled. So this is in our lab at Cern, where we have, you can see the very complicated peripheral electronics board on the left hand side.
STFC-RAL-CR03 R61: So this green PCB, it's actually here. Sorry there's a better quote here we have all the connections possible. So up to 55 here.
STFC-RAL-CR03 R61: this is all of the DC. DC. Converters to distribute the power. Low voltage. We have 9 versatile links for the electrical to to optical transmission on detector, and then we have 12 lpgs on such a board which are on the backside, shielded against the cooling plate. So this is very, very difficult design. In fact, it has 22 layers. If I'm not mistaken.
STFC-RAL-CR03 R61: so it's a massive board, and you think size better. It's very difficult. It took companies in China several attempts to produce this, because it was so challenging, it was great.
STFC-RAL-CR03 R61: Okay, so this is our full scale testing. Now it's certain. And how the detector will look is similar to this on the left. Then you imagine just a full circle
STFC-RAL-CR03 R61: into the Escape Times.
STFC-RAL-CR03 R61: and I didn't mention anything on how we associate time to a track. So if I leave you with this last slide
STFC-RAL-CR03 R61: going beyond, then the technology. But then the physics potential. And this is also an area which is under full development
STFC-RAL-CR03 R61: in terms of how we can use timing information to improve the physics potential of Atlas so clearly in the 4 region. This is quite clear that we can do track by track checks of the time to check if they're from the same vertex or not. So this is what we would call sort of track, cleaning, self timing, sort of
STFC-RAL-CR03 R61: style. There's also a lot of work to try and associate a time to the heart scattered vertex. But, unlike Cms, we don't have full coverage of the detector. We only have the forward region. So we have to look at vertices that have a substantial amount of tracks in the forward region, for example, from vector, books on fusion production, where we have jets which are largely in the forward region.
STFC-RAL-CR03 R61: In fact, you see on the left hand side. Here we are looking at different ways with machine learning techniques to associate a vertex time for such events. And you can see signal versus background. Of course it's very difficult, because you only get about 60%
STFC-RAL-CR03 R61: right of of the of the tracks which are associated to the vertex that come from the art scatter tracks. There's a very complicated problem which will evolve over time. It's still in its R&D phase, but the gains are quite large just to leave you with an impression of why this is important, and where we can go with this. This is, for example, a Vbf selection of jets, looking at the increase in signal over backgrounds
STFC-RAL-CR03 R61: with additional pattern projection. And if we look just at the forward force just looking at the coverage of Http. Even with 20% improvements, we can still gain in terms of signal or background up to 10%.
STFC-RAL-CR03 R61: Then looking at the central board, and then the full events, so we can get games which are much larger, sitting over back on the better. We perform in terms of pilot projection for this type of topology, where about 85% of the jets
STFC-RAL-CR03 R61: will fall within zoom within Http acceptance.
STFC-RAL-CR03 R61: So about 85% of events, for example, will have at least one jet to be acceptance that increases as you go higher to higher. Mj, we have interested in interesting of different things to gauge boson couplings. Here you have Mjj, of about a TV. It's more than 95%.
STFC-RAL-CR03 R61: So it's quite a significant amount to gain here by improving how we use time in the events, and here that I hope I can make you with.
STFC-RAL-CR03 R61: We're still there's still a lot to do here into into where the performance can go.
STFC-RAL-CR03 R61: So the last slide then looking at roadmap, and briefly summarizing what what was just discussed.
STFC-RAL-CR03 R61: Http is there to mitigate the pilot effects and improve the overall object and performance of Atlas in the high luminosity. Lhc. It also will provide the necessary luminosity measurement. There was a lot of R. And D, and I'm showing you, of course, a route which is towards the Atlas factor of Http. But of course, Lgap R. And d continues in many different avenues for many different ideas, for future, for timing in the future, whether we're talking fcc ee or Hh.
STFC-RAL-CR03 R61: Going the route of better better
STFC-RAL-CR03 R61: positional precision, not just time precision. So there are a lot of areas of R&D going in these directions, going away from from hybrids to to Cmos, monolithic detectors. But we've had okay, at least in the course of the R. And D. Program for Ldads for Hgtd, we had a lot of a lot of evolution of the program up to the point we have now, which came from a very open and fruitful collaboration with Rd. 50 and Cns.
STFC-RAL-CR03 R61: And we are now certain that we can achieve less than 50 picoseconds per hit up to 2.5 times 10 to the 15 with 50 micron Lgat sensors. And so now we're really
STFC-RAL-CR03 R61: heading to the full scale system tests and production, which is also a very exciting and challenging aspect of the project. I went nothing into the hybridization challenges the detector. Construction challenges the fair ages. You can imagine any source of noise in such detectors is a big problem for timing. So there's a lot of other areas, not just the Elegan performance, which are quite exciting. And so yeah, as I said, the silicon timing is a very active area of detector R&D, which.
STFC-RAL-CR03 R61: of course, I don't have time to talk about today. But there's a lot of different areas of research here.
STFC-RAL-CR03 R61: Go beyond the scope of of this detective.
STFC-RAL-CR03 R61: Thank you.
STFC-RAL-CR03 R61: Got it.