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Anna Phan | USLHC | USA

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So long, and thanks for all the fish…

Monday, September 17th, 2012

So this will probably be my last post. Adam Davis, a PhD student at the University of Cincinnati, will be the next LHCb blogger for USLHC. You read that right, Syracuse University is not the only US institute in LHCb anymore, this year we’ve been joined not only by the University of Cincinnati, but also the University of Maryland. So look forward to new bloggers from both institutes in the future!

While lots of new people and institutes are joining LHCb, on a more personal note, I will be leaving at the end of the month. Leaving not only Syracuse University and LHCb, but particle physics as well. If any of you have read my bio, you will know that I did a double undergraduate degree, graduating with a bachelors in software engineering as well as physics. I chose to pursue particle physics as I wanted to do research and it’s probably the most computationally intensive area of physics research, so I would be able to utilise my programming skills. Not to mention of course, that it’s also a very exciting and interesting field of research. Now though, having devoted almost six years to physics and spending almost half of that time out here in Geneva, I feel that it’s time to take everything that I’ve learnt working at CERN, and apply it to other areas. And I’ve been lucky enough to have been offered the opportunity to do so at IBM Research back home in Melbourne.

This was by all means not an easy decision to make and I’ve spent most of the past year agonising over it. In research, the logical career path is to continue in research and work towards finding a permanent position either at a university or a laboratory. However, this isn’t possible for everybody. Simply by the numbers, there are fewer postdoc positions than there are phd students and even fewer permanent positions, maybe even none at all where you want to live. So most people do end up leaving research, but the opportunities outside the field tend not to be discussed and appear really daunting.

Now I was extraordinarily fortunate to get this job at IBM Research (seriously, I heard about the lab on a Friday and by the next Wednesday, only five days later, I had an informal offer), and you may say that I’m a special case as I have my software training as well as my physics experience to offer, but particle physics really does train you in many practical transferable skills. We learn how to think critically, we learn how to program, we learn to analyse large amounts of data, we learn how to take a large complex problem and split it up into smaller, more manageable problems, we learn how to handle problems outside our areas of knowledge, and I can’t emphasise this enough because it’s just everyday life for us, we learn how to work in large collaborations with people from different cultures and backgrounds.

The hardest thing about leaving research is knowing what type of job to apply for, where to search for job opportunities and how to apply. There’s a nice website, Beyond Physics, which tries to address this information void. However from personal experience, the best thing you can do is tell people what you are looking for and ask them if they know anything. I know it’s scary topic to bring up, particularly with your supervisors, advisors or employers, but everybody knows somebody who has left the field and where they’ve gone and even if they don’t know how they did it, they can always past along their contact information. And of course, don’t forget your family and friends!

Anyway, I hope you’ve all had as much fun reading my posts as I have writing them and that you’ve learnt a little along the way. And I’ll now turn it over to Adam, your new guide to the exciting world of flavour physics. Welcome, Adam!

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Testing theory…

Sunday, August 26th, 2012

As I discussed a BaBar result previously, it only seemed fair that I spend a post discussing a Belle one. For those of you who only associate the words BaBar and Belle to cartoon characters, they are also the names of two competing \(B\) physics experiments, both of which have finished data taking but are still producing results.

So which Belle result have I decided to discuss today? I’m going to talk about the updated measurement of the \(B^- \rightarrow \tau^- \overline{\nu}_\tau\) branching ratio that was first presented by Youngmin Yook in a parallel session at ICHEP and now can be found on arXiv.

Why would I choose this measurement you ask? Let’s have a look at the Feynmann diagram of the process on the right here. In the Standard Model, the decay can only proceed via the exchange of a \(W^-\) boson and so the branching ratio can be translated to a measurement of \(V_{ub}\), one of the CKM quark mixing matrix elements. However, new physics could significantly modify the branching ratio via the exchange of a new charged particle, like a charged Higgs boson.

An updated result of the branching ratio is even more interesting than that though, because the average of the previous consistent experimental measurements from Belle and BaBar, \((1.67 \pm 0.30)\times10^{-4}\), is higher than the prediction from CKM fit, \((0.733^{+0.121}_{-0.073})\times10^{-4}\) and the Standard Model \((1.2 \pm 0.25)\times10^{-4}\). This is what is shown on the left here, where the blue point is the average of the previous results, and the green area is the CKM fit prediction. Could this be due to new physics?

Experimentally, it is quite difficult to measure \(B^- \rightarrow \tau^- \overline{\nu}_\tau\) decays, due to the multiple undetectable neutrinos in the final state (as well as the one from the \(B^-\) decay, there is also at least one from the \(\tau^-\) decay). In fact, I’m pretty sure that we can’t perform this measurement at LHCb at all.

Belle and BaBar are able to as their \(B\) meson pairs are produced through the well defined process \(e^+e^− \rightarrow \Upsilon(4S) \rightarrow B\overline{B}\) and their detectors cover a larger solid angle, which allows them to make a fairly accurate estimate of neutrinos produced in decays. To the right, here is a plot of the extra detected energy in selected events, where the points are the data, the red dotted line shows the signal, the dashed blue line shows the background and the red solid line shows the total fit. They expect the signal to peak at zero, since neutrinos can’t be detected.

For the full details of the analysis, I encourage you all to look at the paper, here I’m only going quote the result: \([0.72^{+0.27}_{-0.25}(stat) \pm 0.11(syst)] \times 10^{−4}\) and then discuss the implications…

Firstly, does this new result bring the experimental average closer or further away from the predictions? As presented by Mikihiko Nakao in a plenary session at ICHEP, the plot below shows that the new Belle average (bottom blue point) and the new experimental average (red point) are both consistent with the CKM fit and Standard model predictions (pink and yellow bands respectively). So no hint for new physics here…

Secondly, since this result doesn’t seem to point to new physics, what does it say about \(V_{ub}\), the Standard Model parameter describing the mixing between the \(u\) and \(b\) quarks? As presented by Phillip Urquijo, also in a plenary session at ICHEP, below is a comparison of the various measurements of \(V_{ub}\), which has historically been an area of \(B\) physics which requires further investigation. This is because there are two different methods to measure \(V_{ub}\), called inclusive and exclusive, depending on what type of \(B\) decays are used, and there is currently a discrepancy between the two, which people have been trying to understand. And interestingly… the \(V_{ub}\) measurement from \(B^- \rightarrow \tau^- \overline{\nu}_\tau\) is in agreement with both methods…

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Measuring matter-antimatter asymmetries…

Thursday, August 2nd, 2012

I’ve mentioned before that measuring CP violation is important in understanding why we have a matter dominated universe. So far, CP violation has been observed in the decay and mixing of neutral mesons containing strange, charm and bottom quarks and most measurements have been consistent with theory.

However, there is one measurement which has found evidence for significant CP violation in the decays of neutral B mesons, beyond what is expected from theory. In 2010, with an update in 2011, reported an interesting observation: that the number of events containing two positively charged muons is lower than the number of events containing two negatively charged muons. Like-sign dimuons can be produced from the decays of pairs of neutral B mesons, since they can mix between their particle and antiparticle states. A difference between the number of positive and negative dimuons is an indication of CP violation. The observed difference was close to 1% and 3.9σ away from the theory prediction. The analysis could not distinguish between the two different neutral B mesons, \(B^0_d\) and \(B^0_s\), so the difference had to be expressed in terms of two asymmetries: \(a^d_{sl}\), the semileptonic asymmetry of \(B^0_d\) mesons, and \(a^s_{sl}\), the semileptonic asymmetry of \(B^0_s\) mesons.

 
At ICHEP, DØ presented direct measurements of \(a^d_{sl}\) and \(a^s_{sl}\), by looking at the decays, \(B^0_d \rightarrow D^{(*)\pm}\mu^\mp X\) and \(B^0_s \rightarrow D_s^\pm\mu^\mp X\).

On the left, I have made a plot of these three results, comparing them to the Standard Model predictions. You can see that all three results are somewhat inconsistent with the prediction, which could indicate a contribution from new physics.

But of course, DØ isn’t the only experiment that is able to measure these asymmetries…

 
 

\(a^d_{sl}\) has been previously measured by both Belle and BaBar using \(B^0_d\) meson pairs produced by the decay of the \(\Upsilon(4S)\) meson and the results combined by the Heavy Flavour Averaging Group (HFAG).

And… LHCb released a preliminary result for ICHEP, measuring \(a^s_{sl}\) using \(B^0_s \rightarrow D_s^\pm\mu^\mp X\) decays.

On the right, I’ve added these results to the DØ ones, and now you can see that the situation now isn’t as compelling for new physics, with the BaBar, Belle and LHCb results all being compatible with the theory.

 
However, all experimental results are still compatible within two standard deviations, so new results are needed to definitively resolve the issue… Stay tuned to see if this is where evidence of new physics is found!

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If I could turn back time…

Thursday, July 26th, 2012

I’m going to do something different today and discuss a result from another experiment… I saw the result this morning and thought the topic would be an interesting one for a blog post.

So what will I be talking about? Time reversal violation!

You might be wondering why I would consider this an interesting topic, we all experience time reversal violation in our lives, everyday events definitely are not symmetric in time – we can always tell when a video is being played backwards. This isn’t the case in the world of particle physics however, where most interactions are symmetric under time reversal.

So why do we expect time reversal violation in particle interactions? It’s related to the underlying structure of the Standard Model, which relies on interactions being CPT symmetric.

What is CPT you ask? It’s the combination of three other more fundamental symmetries, Charge conjugation, Parity and Time reversal. I’ve described C and P previously and also presented results of CP violation in the B and D meson systems.

Now if we expect the Standard Model to be CPT symmetric and we’ve observed CP violation, it follows that we should also observe T violation.

And this is exactly the result that the BaBar collaboration released this morning, where they report “the first direct observation of T violation in the B meson system.”

I’m not going to go through the details of the analysis, it’s quite clever and complicated, instead here is a set of plots from the paper:


The points are the data, the blue line is the model without T violation and the red line is the model with T violation. As you can see, the model with T violation matches the data much better than the model without.

And voila, here you have it! Observation of T violation in the B meson system…

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ICHEP 2012: Day Three

Friday, July 6th, 2012

Today was another day of parallel talks. There was again six streams of talks where again the topics in each stream varied throughout the day. I would have loved to spend the day in the Higgs stream, listening to the new ATLAS, CMS, CDF and D0 results, however, I spent the day in the heavy flavour physics stream. This was because my talk was scheduled in this stream.


Today, on behalf of the LHCb collaboration, I presented measurements of \(B_s^0\) meson lifetimes. And because I can, I’ll summarise the results I presented for you all. *winks*

First off, I need to give you a bit of background regarding the \(B_s^0\) meson. As the \(B_s^0\) meson is neutral, it can transform, via the box Feynman diagrams on the left to its antimatter partner, the \(\overline{B}_s^0\) meson, and back again. If we look at and manipulate the equations governing the mixing and decay of the \(B_s^0-\overline{B}_s^0\) meson system we find that there are two \(B_s^0\) mass eigenstates, \(B_{s,H}^0\) and \(B_{s,L}^0\), with two different lifetimes, \(\tau_H = 1 / \Gamma_H\) and \(\tau_L = \Gamma_l\) with \(\Delta\Gamma_s = \Gamma_L – \Gamma_H\) and \(\Gamma_s = (\Gamma_L+\Gamma_H)/2\).

We can measure \(\Delta\Gamma_s\) and \(\Gamma_s\) through the analysis of the decay \(B_s^0 \to J/\psi \phi\) and access \(\tau_H\) and \(\tau_L\) from the measurement of the \(B_s^0\) lifetime in \(B_s^0 \to J/\psi f_0(980)\) and \(B_s^0 \to K^+K^-\) decays:

\(\Gamma_s = 0.6580 \pm 0.0054 \pm 0.0066\, {\rm ps}^{−1}\)
\(\Delta\Gamma_s = 0.116 \pm 0.018 \pm 0.006 \,{\rm ps}^{−1}\)
\(\tau_H \simeq \tau_{J/\psi f_0} = 1.700 \pm 0.040 \pm 0.026 \,{\rm ps}\)
\(\tau_L \simeq \tau_{KK} = 1.468 \pm 0.046 \pm 0.006 \,{\rm ps}\)

These results can all be shown as a function of \(\Delta\Gamma_s\) and \(\Gamma_s\) like below. You can see that all the results are fairly consistent, and the experimental combination overlaps all three individual experimental results. It is also consistent with the theoretical prediction of \(\Delta\Gamma_s\).

The measurement of \(B_s^0\) lifetimes and the information they provide regarding \(\Delta\Gamma_s\) is interesting as the value of \(\Delta\Gamma_s\) can be affected by physics beyond the Standard Model…

And that’s it for Day Three of ICHEP 2012 for me. Until Monday everybody!

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ICHEP 2012: Day Two

Friday, July 6th, 2012

Today was another day of parallel talks. There was again six streams of talks, though the topics in each stream varied throughout the day. Additionally, a high school masterclass was run for high school students and a development day for high school teachers.

I mention these because I was asked to give a talk during the development day, introducing the teachers to the main concepts of particle physics. I was really worried about giving this talk as I haven’t had any interaction with high school teachers or students since I was a high school student myself. I also didn’t have time to really practice and refine my presentation. I did one practice talk last night at home, and received so many comments that I ended up completely rewriting the talk overnight. Unfortunately I didn’t have time to practice the final version of the talk, and due to the lack of sleep, got lost a few times during the presentation as well as during the questions. Hopefully the teachers got something out of the presentation, I definitely learnt a lot about presenting to non-experts. A skill that I’m going to be putting into practice very soon, as one of the teachers attending works at my old high school and asked me to go and talk to the students there sometime.

I spent the rest of the day in the heavy flavour physics sessions, which were mostly about searches for rare B, D and Kaon decays. I think I can probably summarise all the talks by saying that nothing was found to be significantly in disagreement of the Standard Model and/or results from other experiments…

And that’s it for Day Two of ICHEP 2012 for me. Until tomorrow everybody!

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ICHEP 2012: Day One

Thursday, July 5th, 2012

Today was a jam packed day of parallel talks. There were six streams of talks: theory, heavy flavour (b, c, and s quark) physics, top quark measurements, supersymmetry searches, neutrino physics, and jet physics. I split my time between the various streams, starting the day in the heavy flavour physics stream, and moving to the supersymmetry, then the top quark physics and ending the day back in the heavy flavour physics stream. I thought I would (very briefly) highlight one talk in each session…

Firstly, Leptonic and semileptonic B decays with taus at BaBar by Guglielmo De Nardo of The University of Napoli and INFN. While he didn’t present any new results (they were presented earlier at FPCP), they are still worth highlighting, as they are in tension with the Standard Model. So what results did he present? The measurements of various branching ratios of leptonic and semileptonic B decays with taus, and their ratios. Specifically, R(D) = BF(B ->D τ ν) / BF(B -> D l ν) = and R(D(*)) = BF(B ->D(*) τ ν) / BF(B -> D(*) l ν) = 0.332 ± 0.024 ± 0.018 which in combination exceed the Standard Model predicted values by 3.4σ. Since the results had been presented before, instead of going into the analysis in detail, he presented the analysis within the 2HDM type II theory and concluded that that particular theory couldn’t account for the results.

Secondly, Searches for direct pair production of third generation squarks with the ATLAS detector by Martin White of The University of Melbourne. I’m highlighting this result because of personal reasons. I worked on one of the results presented in this talk during my PhD. Below is a nice summary of the supersymmetric top exclusions he presented. I don’t really want to go into detail about the plot, except to say that no supersymmetric tops have been seen by ATLAS in the six analyses summarised within it.

Thirdly, Tevatron and LHC top mass combinations by Frederic Deliot of The Centre d’Etudes de Saclay. I’m highlighting this talk to be fair to all the experiments. I wouldn’t want to select a CDF result in preference to a D0, ATLAS or CMS one for example. The top quark mass is an interesting measurement as it can be used, in combination with the W boson mass, to predict the Standard Model Higgs boson mass. And so the more precise the measurement of the top quark mass is, the more precise the prediction is. Also, now that we have found a Higgs-like particle at 125 GeV, we could see whether the three masses are consistent within the Standard Model or not. The Tevatron combination is m = 173.18 ± 0.56(stat) ± 0.75(syst) GeV and the LHC combination is m = 173.3 ± 0.5 (stat) ± 1.3 (syst) GeV. It is interesting to note that both values are systematically limited (the systematic error is larger than the statistical one) so it won’t be easy to improve the results.

And finally, Direct CP violation in charm at Belle by Byeong Rok Ko of Korea University. In doing this, I apologise to the BaBar, CDF and LHCb presentations on the same topic, but I choose the Belle presentation as it contained the HFAG combination of all the charm CP violation results.

I blogged about the observation of CP violation in the charm meson system by LHCb when the result was released last year. Since then, BaBar, Belle and CDF have all measured the same quantities and the plot above is the combination of all the results. Numerically, the average \(\Delta A_{CP} = (-0.74±0.15)\%\) and is ~4.9σ away from zero. Which means the LHCb result has been confirmed and we need a theoretical explanation and complementary measurements from other decays…

And that’s it for Day One of ICHEP 2012 for me. Until tomorrow everybody!

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ICHEP 2012: Welcome

Wednesday, July 4th, 2012

Usually the welcome reception of a conference is a fairly low key affair. You register, you get your conference bag, you have a few drinks and nibbles while chatting to other conference participants. Not the ICHEP 2012 welcome reception though. This one included a broadcast of the LHC Higgs Seminar. This meant that people actually arrived before the reception and there was a fairly long line to register (though we didn’t have to wait as long as those at CERN). The main worry about the ICHEP broadcast was the connection to CERN. During the previous Higgs seminar in December, the webcast stopped working half way through. Luckily this time, everything went well, and we were able to listen and cheer at the announcements from CMS and ATLAS of the observation of a Higgs-like particle at 125-126 GeV.

It was a historic moment for particle physics, a triumph for the predictive power of the theory, not possible with without the hard work of many physicists, both on the LHC and the two experiments. Though, as was emphasised, this is only the beginning in Higgs boson studies. We now need to figure out what exactly this excess is… We certainly live in interesting times!

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ICHEP 2012: On route…

Sunday, July 1st, 2012

Greetings everybody! It’s been a while hasn’t it? Things have been a little busy over the past few months in preparation for the summer conference season. I’ve mentioned in a previous post that there’s a winter conference season. There is also a summer one as well.

The big summer conference this year is ICHEP 2012, the 36th International Conference on High Energy Physics. I was lucky enough to be allocated a talk and I’m super excited to be heading to Melbourne, Australia for the conference. Not only are we expecting some very exciting results, but it’s also my home town and I’m looking forward to seeing my family and friends again.

I’ve actually been looking forward to this event for quite a while. I actually remember the group meeting back in 2008 when we were told that we would be hosting ICHEP. From what I understand, the progress for deciding on a host city for the conference kind of works like the decision to host the next Olympic Games. Except instead of the IOC, it’s the IUPAP who decides. And there’s much less scandal involved…

Anyway, since then, every time anybody in particle physics has mentioned that they would like to come to Australia, I’ve told them to come to ICHEP 2012. And it’s been my not-so-secret goal ever since I started this position with Syracuse on LHCb to come back and attend the conference. And here I go…

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Needle in a haystack

Thursday, May 10th, 2012

We are back to discussing B physics today, with the observation of the rare decay: \(B^- \rightarrow \pi^- \mu^+ \mu^-\). So what is this decay? It’s a \(B^-\) meson (made of a b and an anti-u quark) decaying into a \(\pi^-\) meson (made of a d and an anti-u quark) and two muons. And why is it so rare? Well, it’s a flavour changing neutral current decay. Which means that there’s a change in quark flavour in the decay, but not charge. This type of decay is forbidden at tree level in the Standard Model and so has to proceed via a loop, which can be seen in the centre of the Feynman diagram below.

If you look closer at the loop, you can see that for the decay to occur, a b quark needs to change flavour to a t or c quark, which then needs to change to a d quark. This is another reason why this decay is so rare. Transitions in quark flavour are governed by the CKM matrix, which I illustrate on the right, where the larger squares indicate more likely transitions. So while the transition from b to t is likely, the transition from t to d is very unlikely, and the b to c and c to d transitions are both fairly unlikely. This means, that whichever path is taken, the b to d quark transition is very very unlikely.

Okay, now to the LHCb result. Below I have a plot of the fitted invariant mass for selected \(\pi^-\mu^+ \mu^-\) candidates, showing a clear peak for \(B-\) decays (green long dashed line). Also shown are the backgrounds from partially reconstructed decays (red dotted line) and misidentified \(K^-\mu^+ \mu^-\) decays (black dashed line). Candidates for which the \(\mu^+ \mu^-\) pair is consistent with coming from a \(J/\psi\) or \(\psi(2S)\) are excluded.

We see around 25 \(B^- \rightarrow \pi^- \mu^+ \mu^-\) events and measure a branching ratio of approximately 2 per 100 million decays. This result makes this decay the rarest \(B\) decay ever observed!

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