Quantum Diaries

The LHC has restarted this year, with a couple of differences, the most obvious being the energy change from 3.5 TeV per beam to 4 TeV per beam. However there are lots of more subtle changes between this year’s and last year’s running. One of these is the collision setup at Point 8, where LHCb is located.

I should probably pause here to warn you that this post is going to be a little technical. But hopefully you’ll get something out of it, even if it’s only that the LHC is a very complicated machine.

In most of the LHC, there are two separate beam pipes, one for the clockwise beam one and one for the anticlockwise beam two. These two beam pipes can be seen in the above image. However, since we want to collide the two beams, there are sections of the LHC where there is only one beam pipe. As seen below, these naturally are at each of the points where the experiments are located. In these regions, the beams are kept physically separated using magnets (horizontally at CMS and LHCb and vertically at ATLAS and ALICE) and brought into collision at a finite crossing angle to avoid unwanted collisions.

This is where it starts getting more complicated… As I’ve mentioned earlier, each experiment has some sort of magnet system, which could affect the trajectory of the circulating beams. The LHCb dipole magnet produces a deflection of around 180 μrad at the top energy of 7 TeV per beam. The field direction is in the vertical plane and the deflection therefore in the horizontal plane. This deflection must be compensated for to ensure beam stability. The situation can be seen in the image below.

For our physics analyses, we would like to take data with our dipole magnet in both directions (the N pole at the top and S pole at the bottom, and vice versa). We would also ideally like the crossing angle between the two beams to remain the same in both situations to reduce the errors on our results.

The problem here is that both the LHCb dipole compensation and the beam separation and crossing are in the horizontal plane. For the LHC, the beam exchange should always occur in the same direction, that is, the clockwise traveling beam one (the blue one) should always cross from the outside beam pipe to the inside at LHCb. This means that when we change our dipole polarity, we can’t ask the LHC to switch the beams around so we get the same crossing angle in both directions.

So for the 2010 and 2011 LHC runs, we’ve been running with different crossing angles when we switch magnet polarities. This year however, a new scheme has been proposed, where instead of horizontally separating the beams at the LHCb collision point, the beams will be vertically separated. With the vertical separation, and the horizontal compensation, this actually creates a tilted crossing plane, which you can see in the image below.

Now when we change the direction of our dipole magnet, we just change the direction of the tilted crossing plane (from NE-SW to NW-SE), but the crossing angle remains the same. So this year’s results will be even better than last year’s!

At the end of my last post, I left you all with the above plot (from this ATLAS conference note) without any real explanations. It’s actually quite a nice result, so I thought I might go through it in a little more detail today.

So what does the plot show? Reading the axes, it shows the lepton charge asymmetry as a function of lepton pseudorapidity of leptonic W events.

What does this actually mean? To answer this, let’s go back to what a W boson is. On the right here, it’s a cute little plush toy, which you can buy from Particle Zoo. In real life, it’s massive charged elementary particle. This means there is a positive W boson, and a negative W boson, \(W^+\) and \(W^-\) respectively. When a W boson decays into a charged lepton and corresponding neutrino, due to charge conservation, the charge of the lepton must match the charge of the W boson. So the above plot of lepton charge asymmetry is actually a plot of W charge asymmetry, which can be interpreted as a W production asymmetry, \(A_W = \frac{\sigma_{W^+} – \sigma_{W^-}}{\sigma_{W^+} + \sigma_{W^-}} \).

So why is there a W production asymmetry? Let’s look at how a W boson is produced in a proton-proton collision. On the left here, we have a Feynman diagram of this process, where you can see that to make a positive W boson, you need a certain combination of quark and antiquark, most often an up quark and an antidown quark. To make a negative W boson, you need the opposite combination, a down quark and an antiup quark.

The production asymmetry occurs because, as illustrated in the diagram on the right, the proton contain two valence up quarks and one valence down quark in a sea of quark-antiquark paris and gluons. So in a proton-proton collision, there is a higher probability of a up and an antidown quark interacting than an antiup quark and a down quark, and hence more positive W bosons are produced compared to negative W bosons.

So that’s why there’s a W production asymmetry, but why does it depend on pseudorapidity? And what is pseudorapidity anyway?

Well, pseudorapidity is a measure of the angle at which the W boson was produced, which depends on the momentum of the two quarks from which the W boson was produced. The quarks and gluons within a proton carry a fraction, \(x\), of the total proton momentum, which is described by a parton density function \(f(x)\). The plot on the left shows the proton parton distribution functions for various types of quarks and anitiquarks, as well as gluons, for a particular proton collision energy scale \(Q\).

So the momentum of the quarks which produce the W boson varies from collision to collision, depending on their parton density functions, which causes the W production asymmetry, caused by the quark content of the proton, to vary with pseudorapidity. Which is what the plot shows!

Today, I’m going to be talking about some lesser known LHCb results. In fact, I’m going to discuss physics that some people thought LHCb couldn’t do, given the detector and software design.

What am I going to be talking about? Electroweak physics. Yes, you read that right, not the heavy quark physics which LHCb was designed and built for, but electroweak physics. In particular, I’m going to discuss some of our new results on Z and W boson cross sections, which will be presented at the DIS workshop in Bonn this week.

But before I go into the results and why they are interesting, let me quickly introduce the Z and W boson, as found at The Particle Zoo. Theorised in the 60s and discovered in the 80s, they are massive elementary particles that mediate the weak force. Z bosons are neutral and decay into a pair of leptons or quarks. W bosons are charged and decay into either a charged lepton and neutrino or two quarks.

At the LHC, Z and W bosons are usually identified by their leptonic decays. The signatures that electrons, muons and tauons leave in the detectors are much easier to find and measure than those left by quarks. In LHCb, we are able to detect Z decays to a pair of electrons, or muons or tauons and W decays into a muon and corresponding muon neutrino. Unfortunately, we aren’t able to cleanly identify W decays to electron or tauons and their corresponding neutrinos.

Above I present a summary of all the Z and W cross sections we have measured so far using data from 2010. On the left are the Z cross sections, given separately for each decay mode, while on the right are the Z and W to muon cross sections and various ratios of them.

If you are used to seeing LHC results, these may look a little strange. Usually the data is shown as black solid points while the theory is shown as coloured bands. Here the data is shown as the coloured bands, while the predictions of various theoretical models are shown as black open points.

Why this confusing presentation you ask? Well, that has to do with why we are trying to measure the Z and W production cross sections in LHCb.

As I’ve mentioned before, LHCb has a unique geometry compared to the other LHC experiments. In particular, with our cone geometry, we cover the forward region of 1.9 < y < 4.9, while ATLAS and CMS cover |y| < 2.5 with their cylindrical geometries. In terms of proton-proton collisions and the production of Z and W bosons, this means we are able to probe a complementary region of phase space. The plot on the right illustrates this, where you can see that LHCb is able to explore the low-\(x\), high \(Q^2\) region inaccessible by other experiments (past and present). This is important as this is the region where there is the highest uncertainty in the theoretical predictions in the Z and W production cross sections. So ideally, we would like to use experiment to constrain the theoretical predictions.
I say ideally, as if you look at our current results, we don’t have the experimental precision to do this. But we will in the future, so be on the look out!

Of course we aren’t the only experiment looking at Z and W production cross sections, ATLAS and CMS are as well, so I feel obliged to show you this plot on the left, which is of the W lepton charge asymmetry as a function of lepton pseudorapidity from ATLAS, CMS and LHCb…

LHCb is in somewhat of a unique position in that it has no direct competition. No other experiment is currently able to perform all the physics measurements that are possible at LHCb with comparable precision. However, there are a few measurements where there is fierce competition, and one of these is the search for the decay of a \(B_s\) meson into two muons, \(B_s \rightarrow \mu^+\mu^-\).

This particular decay is extremely interesting as it is a flavour changing, helicity suppressed decay, meaning that it is very rare. In fact, in the Standard Model, the expected decay rate is \( (3.2 \pm 0.2) \times 10^{-9}\), meaning that only 3 out of a billion \(B_s\) mesons is expected to decay into two muons. Because the decay is so rare, it provides a powerful indirect way of discovering new physics. If we observe that the decay rate is higher than what is predicted, we have found something! Maybe even Flip’s supersymmetric penguin!

Unfortunately for the supersymmetric penguin, so far all searches for this rare decay have seen nothing. However this year, for the first time, experiments have the datasets to start detecting a signal, even if it’s as rare as the Standard Model predicts. Not only is LHCb sensitive to the decay, but so is ATLAS and CMS, as well as CDF and D0.

And in true competition, ATLAS, CDF, CMS and LHCb have all released new results in time for the winter results. Now, before you all get too excited, there hasn’t been an observation yet, so all experiments have set new upper limits:

• CDF: \( 3.1\times10^{-8}\) (10 inverse femtobarns of proton-antiproton collision data at 2 TeV)
• ATLAS: \(2.2\times10^{-8}\) (2.4 inverse femtobarns of proton-proton collision data at 7 TeV)
• CMS: \( 7.7\times10^{-9}\) (4.9 inverse femtobarns of proton-proton collision data at 7 TeV)
• LHCb: \( 4.5\times10^{-9}\) (1 inverse femtobarns of proton-proton collision data at 7 TeV)

There are a few of interesting things to note about the results…

Firstly, all experiments are getting dangerously close to the Standard Model prediction. I say dangerously close, because the limits are so close to the prediction that there probably won’t be any new physics in this decay, which is quite disappointing. The plot below (a modified version from this paper) shows what types of new physics models increase the rate of this decay and how many have been excluded by the measured upper limits.

Secondly, the experiments have fairly similar sensitivities to the decay, despite the varying datasets used in the analyses. The reason for this is a little complicated, since the sensitivity of an experiment to a measurement of decay rate depends on many things. There is the production rate of \(B_s\) mesons, which depends on the energy of the proton-(anti)proton collisions. This is why CDF needs 10 inverse femtobarns of data to be competitive with ATLAS, CMS and LHCb. There is then the efficiency and accuracy of detecting and measuring \(B_s\) meson decay. This is why LHCb only needs a fraction of the ATLAS and CMS datasets to be competitive. Then there are the technical details of how the analyses are performed, which I’m not going to go through here, but they do vary from experiment to experiment as people try different methods to make the most of the data they have.

But of course, the most important point to take away is that LHCb produces the best limit! *winks* Here is an event display for a detected \(B_s \rightarrow \mu^+\mu^-\) candidate. The muons are the pink lines which pass through the entire detector. A zoom in on the vertex region shows that they are displaced from the primary vertex, as would be expected if they decayed from a \(B_s\) meson.

Winter conference season [*] is upon us, which means everybody is busy preparing new results. Today, instead of talking about the physics itself, I’m going to discuss the process around it; namely the procedure which the results of an LHCb analysis [**] need to go through before being released.

There are two ways in which analysis results are released: either through a conference note, meaning it is a preliminary result, or to a paper. I’m only going to discuss the former because I’m currently going through that procedure at the moment, though as a referee of the analysis and not a proponent.

The preliminary result approval procedure is constantly in flux, but currently, it looks something like this:
which is a simplified (and coloured) version of what can publicly be accessed on the LHCb editorial board webpage.

I think the most important points to note are the levels of the scrutiny that each analysis goes through before release. When I say that “everybody is busy preparing new results”, I’m not just referring to the people who are performing the specific analyses which are being released, I also include all the assigned analysis referees and editorial board members, and the physics coordinator as well as interested members of the collaboration, who can review the public notes and attend the approval presentations.

Believe me when I tell you that there have been/will be lot of extra emails and meetings this month due to all the paper and conference note reviews and approval presentations… Here’s looking forward to the Moriond conferences where the new results will be presented!

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[*] Winter conference season for experimental particle physics refers to the cluster of conferences held in February and March every year. The most well known of these are Aspen, Lake Louise, La Thuile and Moriond. Yes, these conferences are held annually at ski resorts. The conference organisers are understanding enough to give participants time to take advantage of the location, with sessions in the morning and evening, but none in the afternoon. I personally call these conferences “skiing conferences”. I have never been to any of them, but I would love to some day. They sound like the perfect combination of work and fun.

[**] I should probably mention that all experimental particle physics collaborations have some sort of publication procedure, most of which involve some sort of detailed internal document, followed by the public document.

Warning for those on slow internet connections: this post contains quite a few large images
which may take a long time to load. Just be patient, I promise they’ll be worth the wait!

I’ve been blogging about LHCb for about eight months now, telling you all about the detector and the physics. If you’ve been following my posts from the start, you might recall that as well as being new to Quantum Diaries, I was also new to LHCb.

Why do I bring that fact up now? Combined with the timetable of the LHC (which operated between March and November last year), this has meant that while I could read about the detector, monitor the data taking and start analysing the recorded data, I had never actually been underground and seen the detector.

So when I found out that Kathryn Grim, of USLHC Communications, was taking a pair of videographers and photographers down, I asked to be part of the visit. Luckily, there was space for me and I had already passed all the necessary training and had all the required access privileges.

I was pretty excited about the visit, in addition to getting to see the detector I work with, the last LHC detector I saw was ATLAS, back in 2009 before any serious data taking had begun. And before that, I visited ATLAS and CMS during construction way back in 2007.

Why is this history important? Well, visiting LHCb is a history lesson of sorts. Unlike ATLAS and CMS, which are located in caverns especially built for the experiments, as seen the schematic map below, LHCb and ALICE reside in caverns which previously contained LEP detectors, DELPHI and L3.

As I’ve mentioned before, ALICE took advantage of that fact by incorporating the L3 magnet in its detector. LHCb took a different approach, simply disconnecting the DELPHI detector and moving it away from the beam line into an exhibition area behind concrete shielding. I didn’t have much time in the DELPHI part of the cavern as the videographers and photographers wanted to get straight to LHCb, but I was able to grab a couple of shots of the detector, one of which I include below…

So you may be wondering about the videographers and photographers Kathryn and I were accompanying underground (along with a couple of other LHCb colleagues). It was kind of confusing actually, there were two separate crews, each of which contained one videographer and one photographer. However, the focus of one team was the videographer and the focus of the other was the photographer.

Here on the left, I have a photo of the videographer, Steve Elkins, who was filming for a documentary. He had a accompanying crew member to assist with the filming and to take photos of the process for promotion. You can find out more about the upcoming documentary at his website.

In his words, “The film will be about questions, and the diverse routes to ask them. It will be about the struggles to lift the seemingly impenetrable veils of mystery from the intangible and transcendent, whether through bodies, machines, brains, or stars… It will involve the largest astronomy project in human history, Tuvan throat singers, a neuroscientist’s quest to actually photograph memories being formed in the brain, and the Kalacakra sand mandala ceremony overseen by the Dalai Lama in India, all told through the true story of a man running alone across Death Valley in average temperatures of 130 degrees fahrenheit.”

It sounds really intriguing and I look forward to seeing it.

Here on the right, I have a photo of the photographer, Enrico Sacchetti. You may be wondering why a photographer requires a videographer. It has to do with the camera he was using, a Phase One 645DF. From what I gathered, the company lent him the camera, on the condition that he film himself using it for promotional purposes.

You can find some of his previous photos of the LHC experiments on his website, which are quite nice. From what I saw on the preview screen on the camera though, the new ones will be spectacular.

That’s enough about the people on the visit; onto photos of the detector! I won’t bombard you with images of the whole detector, since they all look fairly similar, but instead, below, I’ll show you a few different unique views of certain components.

The top photo shows the view between the hadronic calorimeter and the muon system from below the detector, looking up towards the ceiling. You can see the beam pipe on the right of the photo. The left photo shows people working in the tracking system. The experiments use the LHC downtime to maintain their detectors. You can see that two of the tracking stations have been retracted, while one remains in position (the two left stations are the retracted ones). The right photo shows the dipole from the front, with a lot of safety tape and plastic covering the beam pipe. These are placed there during the maintenance period to protect the equipment. They will be removed before the start of data taking so they won’t interfere with the physics.

Pretty cool huh? I really enjoyed my visit and the unique opportunity to witness physics and art in action. I’ll leave you all now with the obligatory photo of me and the detector.