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Regina Caputo | USLHC | USA

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How we’ll find the Higgs – a grad student perspective

Friday, December 11th, 2009

I’ve been reading a lot of the comments to the blogs and realized that it’s a little unclear what physicists mean when we say we’ll find the Higgs. All the bloggers have posted pretty event display plots which are a visual representation of what we get in the detector. You can see lines of tracks in the inner detector, energy deposition in the calorimeters and tracks in the muon system. You’ll also see particle tracks bend in the magnetic field. This is beautiful and shows exactly what the detector “sees” when an event occurs. This, however, is not what physicists will use to discover the Higgs.

So what do you do exactly?

It’s never a matter of finding “one” Higgs and declaring victory. Based on different theoretical models we’ll be looking for different signatures. Each of these signatures will have certain background events. Background events are events that  have the same or similar signature as the signal (or in this case, the Higgs).

higgs_boson

So here’s an example:
One signature we can look for is the Higgs decaying to two Z bosons. Z’s have a nice signature because they can decay into 2 electrons each. (for example… they also can decay into muons, quarks… etc, but this is just one example).

So now we have something to look for: 4 electrons which “come from” 2 Z’s. But we can’t just look for 4 electrons because there are other processes that also decay into 4 electrons. For example we can create 2 Z bosons without coming from a Higgs. This is called diboson production. This would be a background to the Higgs->ZZ signal.

So now what do we do? We have to somehow distinguish the difference between the signal (H->ZZ->4e) from the background (ZZ->4e) because in the data there is a probability that all of them will be produced. Well there are lots of ways we try to do this. One is that there would be a peak at the mass of the Higgs if we reconstruct and combine all the decay particles. In order to get this peak we’d need lots of signal events because it’s about getting enough statistics to distinguish them from the ZZ->4e background. We’d need a signal that’s ~5 sigma above the background to claim discovery. What that means is that it has a <0.01% chance of being a fluctuation in the background.

Although this is just one example of how we’d have a discovery, it’s similar for all discoveries. Some particles are easier to discover because they have a very high signal to background ratio, so it will take fewer statistics to get to ~5 sigma. The Higgs is difficult though, for a multitude of reasons. What this means in the end is that it will take more time to get the statistics to distinguish it from the background.

Then once we’ve been able to find an event that we believe is a Higgs candidate we’ll reconstruct it with the event display software that’s been seen so often on the blogs. I hope this sheds a little light on how discoveries will be made at the LHC. Although I’ve never gone through the process myself, there are lots of physicists (and LHC blogers) that have done this before (like during the top quark and W/Z discoveries). So please feel free to ask questions if you have them.

-Regina

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Saturday night

Saturday, December 5th, 2009

You know you’re a grad student if you spend your Saturday nights watching the Control Room Channel to see some of the first 900 GeV collisions. Although that sounds like a bad Jeff Foxworthy joke, sure enough, here I am with my laptop open and my monkey pants on. The lamest TV ever is on Saturday night, but it doesn’t matter. Word is that there will be collisions this weekend and even though I’m not in the control room, I’d still like to see them live (even though for some reason there’s no sound…). Some of my friends at CERN are at the LAr (liquid Argon) desk. Here’s a sneak picture I got of them 🙂

In the Atlas Control Room

In the Atlas Control Room

Although it’s almost 3 am there, the control room is still full. Normally the control room isn’t this packed, but like a baby’s first steps, the first few collisions are always exciting. Unfortunately about every half hour, I have to refresh the page. I don’t even remember how long I’ve been watching, but it sure is exciting. Good thing there’s a coffee machine in there, and I’ve got my fresh pot here. Ooh winter storm warning for New York, you can’t phase me now… I’m in for the night.

-Regina

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Glee!

Thursday, December 3rd, 2009

Even with all the excitement that’s been happening at the LHC I’ve somehow managed to find time to keep up on my favorite show in the whole world – Glee! (currently on television, that’s not the Daily Show). It was on last night. I get made fun of quite a bit for this one, but I can’t help it. Every week my Wednesdays brighten up a little at 9:00/8:00 central (or on hulu anytime) when I get to watch the kids at William McKinley high school sing about, well, everything. I then get the songs stuck in my head for at least a week, which also brightens my day because the sound track is usually pretty awesome. (Single Ladies week was a bad week though… I still get that song stuck in my head if I think about it).

Unfortunately the fall finale is next week, (thanks, American Idol… grrr) so I’ll need to find a substitute. Since I don’t know any other high school drama/comedy/musicals on TV now, I was thinking I can just add songs to my every day routine. While at work, I’m sure my colleagues will chime in, and maybe even do a dance number with me. When my code won’t compile I can bust out some Frustrated Incorporated by Soul Asylum. When I can’t find a new analysis to work on I feel like It’s All Been Done, BNL. When I’m pulling an all nighter a little Beatles A Hard Day’s Night. If only I could figure how to incorporate some Lady Gaga into the mix… Poker Face is a good blogging song, right? hum maybe it’s a better results presenting song… something to think about for next time.

-Regina

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First Candidate Collision Events in ATLAS

Monday, November 23rd, 2009

A picture is worth 1000 words

atlas2009-collision-vp1-140541-171897

atlas2009-collision-atlantis-140541-171897

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Perspective from the US

Monday, November 23rd, 2009

This week has been particularly exciting for those at CERN and the collaborators. We’re back to circulating beams – which was where we left off last September. I anxiously await news about collisions and soon ramping up the energy. My browser has about 10 tabs open looking at views of the ATLAS control room, the beam info, the event display, twitter 😉 … it’s exciting times, and hard to think about other work.

But this has given me some time to reflect on the past year. I arrived at CERN just after the first beam circulation. The golden week or so between the LHC working and not. There was so much excitement at CERN, scientists are just like big kids (myself included). Beams were circulating – everything was going so well. No one was really prepared for what happened. I can only imagine how things are this year. From my friends who are there, I feel cautious optimism.

After all the roller coaster that was 2008 and most of 2009, I’m back refreshing webpages every 2 seconds, but not as doe-eyed as before. Things never go as well as you hope, especially not cutting-edge machines. They’re in the process of deciding which energy to start colliding particles – the first step. We’ll soon surpass the accomplishments of last September and hopefully start a new era in particle physics and I wait with cautious optimism.

-Regina

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LHC Schedule

Wednesday, November 18th, 2009

Sorry I didn’t include this in my last entry. Literally as soon as I posted it, I got another email about the LHC Schedule… which said so far things are going well and that the restarting of the LHC is imminent. That means beams should be circulating soon. Things are changing very quickly and as things are happening we’ll try to keep you up-to-date.

For the public, there will be some quasi-live event displays will be posted here

Once there are events, you should be able to see them. So check it out 🙂

-Regina

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Jamboree at BNL

Wednesday, November 18th, 2009

This week I’m attending an analysis Jamboree at Brookhaven National Lab. When I was little, the word Jamboree always conjured up images of country bears playing banjos (maybe that’s because my grandparents would take us to Disneyland…).

Country Bear Jamboree

Country Bear Jamboree

Unfortunately this Jamboree doesn’t include singing bears, instead it’s a discussion of different analyses to do with the first data from the ATLAS. BNL is one of the major hubs in the US for ATLAS, so about twice a year they host analysis meetings. Different running conditions sometimes warrant different analyses and with data coming hopefully soon, we need unite our efforts and make sure all the things – like software – is standard.

A bit about calibration

I’m giving a presentation on a calibration study I’ve been doing. Calibration is one of the first things we’ll have to do with first data. Like any tool, we’ll need to make sure we understand what we’re getting out of the detector once the data starts rolling in. It’s not as glamorous as a search for an unknown particle, but it is definitely important.  Particles like J/psi and Z have distinct mass peaks (at 3 GeV and 91 GeV respectively) when the energy from their decay products is reconstructed and combined. So we’ll take the reconstructed electrons, and look for a peak at around the Z mass and then tweak our algorithms so the peak lines up with the known peak. This type of calibration for example can be done using only one piece of the detector (like the calorimeter).

Another type is calibration between detector pieces. I’m looking at a study which compares what you read as the energy in the calorimeter and the momentum in the tracker. If you take the ratio, you should get about one, (since the mass of the electron is so small… you all remember E^2=(pc)^2+(mc^2)^2, right?)

But other than calibration

For lunch today we ventured off the BNL site to eat at a staple in American cuisine: Taco Bell. My friend and former roommate was back at BNL from CERN to participate in the Jamboree festivities, so we were celebrating America by getting refills on our super-sized drinks and cheesy Gorditas (sure I know what you’re thinking… Taco Bell is “Mexican” food… right). The culture shock is always a little surprising when going to CERN or coming back.

So now it’s after lunch, so back to work.

-Regina

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LHC Blog Enthusiasts

Wednesday, November 11th, 2009

For all of you who enjoy reading our blogs on the US LHC Blog site or on Facebook, we now have another way for you to access our posts via twitter.

So if you have a twitter account: follow us! The name is US LHC and the site is: http://twitter.com/uslhc

Also check out the latest article in Symmetry Breaking about the LHC start up and how accelerators are the key to national prosperity. The folks at Symmetry Breaking always write interesting articles about the status of particle physics so it’s a great way to keep in the know. And for those of you on Facebook, you can get updates whenever there are new articles if you become a fan. So I encourage you all to sign up!

-Regina

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The Joys of Submission

Tuesday, November 3rd, 2009

A couple of months ago I blogged about how at ATLAS we’re using cosmic rays to study the detector.  Well with data impending, the work that my group and I have been doing was submitted to become internal ATLAS document last week. This process was new to me… so I thought I’d share. Not every plot we make is available for public viewing. Our “notes” (ATLAS documents) come in two flavors: one that is available for public view, and one that isn’t. The ones that aren’t for public view don’t really have any special information – for ATLAS eyes only – they just don’t require that all the plots included are approved by the group they’re associated with. For example, the study I did was on the uniformity of the Liquid Argon (LAr) Calorimeter, so the LAr group has to approve the plots (in other words, make sure they aren’t confusing, that things are labeled properly, that it’s relevant… etc). The process of writing this note took about 5 months. There were at least 5 direct authors and about 10 total people reading and giving input (relatively small group for ATLAS standards 🙂 ). However, with every plot, and with every paragraph we had to make sure we understood exactly what we were implying. From new questions came more cross-checking and new understanding. There was one point where we were comparing numbers of events in a specific region of the detector because I wrote my code separately than the other members of the group (which is also standard.. being able to check independently).

We had several plots approved, but wanted to include a couple of additional plots for clarity – which means that it’s internal only. So finally after all this – months and months of back and forth, editing and re-editing, we submitted… as a “communications” note. This means that it hasn’t been reviewed yet. Before it can be an “internal” note, it has to be checked by an independent group. Then we make changes as suggested, and it can finally be approved. This whole process reminds me of that old school house rock song about how a bill becomes a law. See there’s me waiting for the note to be approved.

There we are waiting for our note to become official

There we are waiting for our note to become official

So Tuesday was a day for celebration (ok, so we don’t need too much of an excuse to celebrate). But some of the plots we worked on are going to go into another publication, which will be public and I’ll definitely share when it’s available. That is a whole other process though for another time.

-Regina

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Sunny Sunday

Sunday, October 25th, 2009

This weekend the weather has been playing tricks on me in New York. I intended to go camping upstate with some friends, but after a valiant attempt on Friday evening/Saturday morning, we decided to take our soggy sleeping bags and head back to Long Island. It literally rained all day, night and the next morning – which being from Colorado – I’ll never get used to. I decided to share this with you on a Sunday afternoon sitting in my apartment looking out my window at this:

A room with a view

A room with a view

The gods must be conspiring against me to make sure I get work done this weekend :).  So I thought I’d update everyone as to the status of the LHC. My email’s been a buzz with information. So far all the repairs have been completed and the entire ring is back at the operating temperature of 1.9 K. The schedule is still on to start circulating beams in mid November with low energy collisions soon to follow. Although we probably won’t be at the full energy this year, any collisions would be an amazing milestone.

There’s also a new LHC First physics Physics Website that you will probably want to check out. It will have the most up-to-date information. Happy reading on a beautiful Sunday!

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