Report from DIS 2013

The series of workshops named “Deep Inelastic Scattering (DIS)” started way back in Durham, UK in 1993. In the last twenty years, particle physics has evolved in many ways, and this years DIS held at Marseille between April 22-26th was a testament to that fact. While it was one of the biggest conference in terms of Standard Model physics talks from ATLAS, it included talks and latest results covering the full ATLAS (and other big LHC experiments) physics program.

However, the big difference from many other conferences was the presence of many colleagues from smaller non-LHC experiments, presenting their exciting results. The program was divided into different, but often closely related working groups, coverings structure functions, small-x phenomenon, electroweak searches, hadronic final states, heavy flavour, spin physics and future DIS experiments. The first day was all overview plenary talks, while the next  few days we had all the working group sessions running in parallel. Finally the last day saw a summary from each working group conveners and the conference concluded with a road-map talk by Rolf Heuer, the director general of CERN.

DIS-type results are crucially important from PDF fitting, and the first working group focused mostly on that. It was also interesting to note that LHC-data is already finding its way into the fits. The second working group featured a multitude of interesting soft-QCD and diffractive physics results from different HERA, Tevatron and all 5 LHC experiments including TOTEM. This working group had a strong overlap with the jets working group, where the focus was on QCD measurements from LHC experiments, HERA and B-factories. The electroweak working group covered latest measurements as well as Higgs and beyond the Standard Model searches. The heavy flavour group also had overlap with structure function and jets working groups, and included results from LHC experiments, Tevatron, HERA, BELLE, BABAR and RHIC, as well as many theory talks. The spin physics group featured results from many specialised experiments like Compass, Hermes, JLab,, as well as from BABAR, STAR and PHOENIX and many relevant theory talks. The last working group focused mostly on the design and prospects of LHeC, which is a proposed colliding beam facility at CERN, using the LHC setup for lepton-nucleon scattering.

Marseille is a charming port city in the south of France, and generally excellent weather and great food contributed to a productive meeting. However, we did miss not having the local specialty “bouillabaisse” in the otherwise great conference dinner!

Many pictures from the conference can be found here.

Deepak Kar is a research associate in Glasgow. He is involved in soft-QCD measurements, Monte-Carlo tuning, and jet substructure studies.

Moriond EW feedback

“Moriond”, that’s an important keyword in our collaboration. It’s the winter deadline for many analyses, the occasion to see first results with the whole set of data collected in the past year. An important conference, one of the milestones of the year.

But there’s more actually: theorists and experimentalists brought together in an atmosphere favoring exchanges and discussion, confirmed and well-known scientists listening to young students presenting their work, and much free time to enjoy resources of the Italian Alps mountains (the conference is held in the ‘La Thuile’ winter sport resort). I’ve not attended many conferences myself, but if you ask me,  free time between noon and 5PM is definitely a good idea!

I had the great privilege to attend the electroweak session this year, and to present the analysis I’m involved in. I’m very grateful to my teammates for all the hard work, with a special mention to the coordinator of the analysis who pushed us to make it in time… For the actual presentation, I had five minutes to go over the full analysis, just enough to highlight a few ideas and results. I was a bit skeptical at first, but the format works actually well, and the young scientists sessions, jumping from one topic to another every 5 min, were quite animated and very interesting.

“Winter deadline” I said. Well, we got a full review of state-of-the-art particle physics, and an impressive set of new results from particle physics experiments. Unfortunately, despite the amazing variety of searches that are performed all over the world, the Standard Model managed to go through the conference without getting a scratch. Though, for those who really didn’t want to return home with empty hands, we got a few curiosities, and we look forward to the follow-up.

As an example, the Fermi-LAT collaboration presented results for the analysis of 4 years of data taking. This telescope is scrutinizing the sky at an impressive rate (20% instantaneous coverage, full scan in 3 hours), looking for WIMPs. After all, dark matter is currently one of the best experimental evidence that something is missing… One of the flagship analysis is the search for a ray in the energy spectrum of incoming photons. That could in particular be a sign of WIMP-like dark matter, that may produce this kind of rays in 2-to-3 co-annihilation processes, with a Bremsstrahlung photon.

Fermi-LAT photon energy spectrum

A 3-sigma bump had been seen last year by an independent analysis around 135 GeV (after correcting for look-elsewhere effect), and raised lots of excitement.  Well, the ‘official’ analysis with the 4 years data confirmed this bump, below 2 sigma after corrections. This sounds promising, but: a control sample built from residuals of cosmic rays interaction with Earth’s atmosphere, shows a line-like feature in the same energy region… So, the bump may as well be an artifact, potentially attributed to changes in event selection efficiency . More investigations are ongoing, and we hope to have the final word soon…

Another curiosity has been shown by the IceCube experiment, where they observed two neutrino showers at an energy of the order of 1 PeV… These are by 3 orders of magnitude “too soft” to be signal (GZK neutrinos), but current models predict only 1 event per year at the tenth of this energy, so this is quite intriguing. Here again, looking forward to more data!

As a wrap-up, I can only encourage you to take a look at the experimental and theoretical summaries of the conference, that would give you a more fair idea of the set of results that were presented. I’ve definitely enjoyed my participation to this event, and if I can give an advice to my fellow PhD colleagues: do your best to attend next year’s session!

Julien Maurer Julien Maurer is completing his PhD at CPPM, Marseille (south France). He has been involved in electron performance measurements and is currently looking for hints of strongly produced superpartners in lepton pairs.

Japanese Delights at HCP2012

This is the last of the Hadron Collider Physics Symposia.  I learned that during the first day of HCP2012.  Its spirit, thankfully, will continue, as HCP merges with PLHC (Physics at the LHC), to form a conference series entitled LHCP(Large Hadron Collider Physics Conference).

I prefer these sorts of conferences.  There are over 200 participants, which ensures that there are a good number of people from a wide variety of experiments, working on a diverse range of research topics, from theorists and experimentalists from the LHC and the Tevatron.  But they’re not too large, and are able to retain an intimate atmosphere.

Impressions of Japan

Kyoto is an awesome place to hold a conference!  Aside from the physics, I was really looking forward to visit this area of the world.  Being of Taiwanese origin, I am keenly aware of the deep cultural ties existing between Japan and Taiwan, and so visiting here was somehow connecting me to my roots.  The few phrases of Japanese that my grandparents taught me (they were educated in Japanese) definitely came in handy!  With my East Asian appearance, however, it wasn’t obvious to the Japanese that I actually don’t speak their language…and I contemplated ways to make that more obvious: maybe I could dye my hair blond, but that wouldn’t help, since many of them do that themselves!

Kyoto really exhibits the contrasts of Japan well.  From traditional Japan, one is awestruck by the beauty of the many temples, with their picture-perfect gardens nestled in front of the sublime backdrop of the surrounding mountains. The autumn foliage just adds to the beauty and elegance these scenes provide. My favourite is the serene Shoren-in temple (whose garden is shown), frequented slightly less by the tourist hordes….


As one comes back into the city, modern Japan, with its hustle-and-bustle, neon lights, and an entire populace armed with smart-phones, one really perceives the differences between these contrasting worlds of traditional and modern.  The prevalence of heated toilet seats is a testament to how ingrained technology is in this land.

It’s also a joy to be among the Japanese people.  They are unbelievably polite and courteous. And as foreigners in their country who don’t speak their language, I feel very awkward when they are the ones appearing apologetic for not speaking better English to me……”no, it’s I that must apologize for not speaking any Japanese,” I wish I could reply!

Victory for the Standard Model

The first day of the conference (Monday) kicked off with a very nice result from LHCb, the first experimental evidence of B_s ->μμ decays.

Below: CLs upper limits on the B_s->μμ branching ratio, as a function of the branching ratio itself.  One sees clearly that the observed limits correspond to the SM prediction+background hypothesis, and that the result is inconsistent with the background only hypothesis.

This is a process that has been looked for over many decades, and this result is really a milestone in the field of B physics.  This particular process is sensitive to new particles beyond the Standard Model, and deviations of the branching ratio from the Standard Model prediction would have pointed strongly to the existence of non-Standard Model phenomenology.

Seeing the first experimental evidence for this process is indeed exciting, but it was a bit of a pity that the measured value corresponded very well to the Standard Model value.

As many searches for new phenomena were presented on Tuesday, one couldn’t help being a bit disappointed, as limit after limit was presented, with no sign of any physics beyond the Standard Model.  Note that it was great to see such a variety of searches, with ever increasing discovery reach, and I wasn’t disappointed at all with the quality of these analyses.  But it would have been nice to see some sort of effect that would have gotten the room abuzz……..

Below: CMS limits on a heavy resonance decaying to lepton pairs, as a function of the resonance mass; and ATLAS limits on 3rd generation Leptoquark production as a function of Leptoquark mass.







But I think one just needs to be patient with the quest to discover something beyond the Standard Model.  After all, the LHC is a huge project that spans many decades, and will have many more years of operation to come.

A new era for Higgs boson measurements

In fact, I was amazed by how much sensitivity ATLAS and CMS now have to the new Higgs boson-like particle with a mass of 126 GeV.  From talks on Wednesday and Thursday, it was clear that we are in a new era, going from discovery mode of this new particle to a time where we will be looking at measuring its properties with ever increasing precision.  There will be certainly much we will be able to learn from this new creature in the particle physics zoo.

So even with Standard Model predictions holding strong thus far, there is still much to explore, and many more exciting results in the future to come!

Last thoughts

Usually, five days of talks at a conference are more than enough for me.  For the first time, I found myself wondering why time passed so quickly during HCP2012. The quality of analyses, and the sensitivity of the experiments with the data they have accumulated is mind-boggling.

Below: Okonomi-yaki, delicious Japanese goodness!

And Kyoto was the perfect backdrop to such discussions. Where else could one mull over the implications of these results, whether over Oyako-donburi, Nigiri, Tempura, or Okonomi-yaki?  What a shame to leave the land of the rising sun so soon………

To the HCP conference organizers:



Stan Lai Stan Lai is a Canadian in Germany, and enjoys hiking, cycling, and looking for good traditional local food wherever he happens to be at the moment. In his spare moments, he also dabs in experimental particle physics research with the University of Freiburg.

TOP 2012 – Part 2

Welcome back, dedicated top quark enthusiast. I’m sure you’ve all been waiting on the edge of your seats for an update from TOP 2012, and I can now confirm that a combined team of LHC & Tevatron physicists narrowly beat a mixed team of physicists from LHC & Tevatron at croquet.

Congratulations to both colliders on their wins and losses, and to the theorists, whichever team you ended up on! How many jokes were there about ‘conservation of angular momentum’ and ‘pileup’ you might be wondering? Too many, and we’ve all agreed not to speak of it again.

A big mass with a small error

‘Finally, some physics,’ I hear you cry! I will surely be doing an injustice to many by singling out any one result, and as a young PhD student I probably won’t even pick out the most significant, but I can’t help but look at the LHC and Tevatron mass combinations and think, ‘Cool!’

Performing a top mass measurement is far from easy, even in the most agreeable of decay channels. Presented at the conference were nothing short of 20 different mass measurements[1] that not only have overcome the huge challenges unique to each analysis, but have managed to combine these results to obtain a measurement of staggering sensitivity. This may sound easy to the uninitiated but when done correctly these combinations are a vast undertaking.

The Tevatron plot you can see above has an uncertainty of less than 1% on the final combination. One percent! Perhaps even more exciting is that the Tevatron hasn’t finished analysing all of its data. Many of these results will be updated to the final dataset, and of course the LHC is catching up fast! Perhaps if we all donate the required offerings of coffee and Red Bull, and pray to the gods in charge of the grid, we may just see a world combination before too long.

World leading measurements!

Let’s face it, we all have a barely completely justifiable sense of loyalty to our experiments. But top quark physics is a broad field, and it would be criminal not to point out a few of my favourite highlights from all of the experiments.

ATLAS: Spin correlation

Convenors would probably argue about this choice for ATLAS, but I think it’s a great little analysis, with some really talented blogger/physicists working on it. These types of angular distributions are excellent probes for non-resonant new physics and a lot of attention has been paid to them at the conference. Though I picked out ATLAS, new results are also available from CMS (also pictured), and DZero and CDF are well worth a read.

CMS: W associated single top production

Hot off the press, this analysis was approved just in time for this conference. Soon to be submitted to Physical Review Letters (PRL), this cross section represents the best known evidence for the W boson associated single top production channel. This channel plays an important role in many analyses and understanding this production mechanism will be crucial in later results.

DZero & CDF: Harder, better, faster, stronger, forward, backward!

When I asked a DZero colleague which was his favourite analysis, he laughed and bought me a beer. I took this to mean that he likes them all and therefore I picked my favourite. Combined with CDF, both experiments provide fascinating results on one of the most interesting anomalies observed at the Tevatron, the top forward backward asymmetry.

This is a phenomenon unique to the Tevatron collider[2], as the Standard Model predicts practically no asymmetry. Nevertheless, CDF and DZero have consistently measured an unexplained asymmetry for several years and the question on everyone’s minds is: Could this be an effect from new physics? Time will tell…

TOP 2013:

Hopefully you now have a nice overview of what has been a fantastic conference. There were far too many results to do them all justice, but if you’re after a detailed description of each talk (presented in a sarcastic, British style), then feel free to fly me out to your institute or nearest luxury spa/resort at your convenience. For those not blessed with a limitless travel budget, find your nearest top quark physicist and ask them to explain it to you over a beer.

If you wanted a ‘what to look out for’ summary, personally I’d say angular correlations, mass combinations, and interplay with the Higgs sector. Of course that’s from an experimentalist point of view and I’ve barely touched on the theory. Keep an eye out for the public results, and for those with Indico access you can find the slides from all talks here.

We now have a long year to wait until TOP 2013, and I for one am counting the days. See you all in Durbach, Germany!


[1] Tevatron – 12 (3 new), LHC – 8 (4 new)

[2] The LHC is a charge symmetric proton-proton collider and therefore has no ‘forward-backward’ asymmetry. One can define a ‘forward-central’ asymmetry that probes the same physics and a great deal of attention is paid to this at ATLAS, CMS and now also LHCb.


James Howarth James Howarth is an experimental PhD student studying at the University of Manchester, UK. His physics interests centre on top quark physics in general and on top properties in particular.

TOP 2012 – Part 1

Greetings from the TOP 2012 conference, Winchester UK! What’s a ‘Winchester’ I hear you asking? A type of gun? Indeed yes, though sadly not of the smoking variety that we’re all so keen to find. However in this particular case Winchester is a historical town in the south of England, complete with the typical rolling green fields, a cathedral, and the not so typical contingent of visiting physicists!

This is the highlight of the conference year if you’re a top quark physicist. Better than ICHEP (unless of course, you actually got to go to ICHEP) and more relaxed than Moriond QCD. Five full days in a four star hotel in the English countryside with only the top quark on our minds, bliss! There’s also a pool and a spa, but who cares about such trivialities…

The best ‘little’ particle there is!

In the year of the Higgs discovery, what might we expect to see in the top sector? Well intrepid reader, that’s a mighty fine question and one that has an agreeable sense of symmetry to it. Just as we’re beginning to probe the boson responsible for bestowing particles with mass, we’re well and truly entering into an era of precision measurements of the standard model’s (SM) most massive particle! In this year’s conference all four five[1] hadron collider experiments involved in top quark physics will present cutting edge, state of the art results on everything from cross sections and properties, to resonances and exotic particle searches. This aptly named particle is, for lack of a more articulate argument, AWESOME [2]!

Conference goody bag win!

Now it’s only the first day of the conference, and we’ve plenty of reason to expect some very exciting results over the next few days, but what has got me so excited on day one? Is it the promise of some ‘friendly’ games of football and croquet with the theorists? No of course not, it’s the simply amazing conference goody bag, in which you’ll find this little gem!


Not only is it a laser pointer (never again will I have to gesture wildly from the audience at a dubious data point on a plot that the speaker pretends not to be able to see), it is also a more retro mechanical pointer, with a pen on the end as a finishing touch! Take that ICHEP!

Why do we care?

So why should you care about top quark physics? If the particle found is indeed the fabled Higgs Boson, then the top quark is the highest mass particle known to exist and may play a special role in electroweak symmetry breaking. Furthermore, there’s barely a new physics model out there that doesn’t involve the top quark in one way or another and top events are fantastic probes for new physics particles such as Z’, axi-gluons and certain flavours of SUSY.

Aren’t you going to show us some plots?

Since many of the physics analysts are taking advantage of last minute editorial board approvals, some of the more interesting and controversial plots are not available for me to post (at least not yet). But there are a few highlights that we can be pretty sure are on the way. Almost certainly there will be a lot of interest in the latest and greatest LHC top cross section combination and time will surely be allotted to the most current forward-backward asymmetry results from CDF and DØ. But with the full 7TeV data sets under their belts, and 8TeV on the way, it will be exciting to see what ATLAS and CMS have been doing with the forward-central asymmetries and lepton charge asymmetries.

If you’re not familiar with these terms, I could easily spend several blog entries explaining why this is a really cool measurement, and why it tantalisingly offers hints of beyond the standard model physics. But I suspect if you’ve read this far on a blog entitled “TOP 2012” then you’ve probably already heard of it. For those that haven’t, the following link is a very nice summary paper.

It’s a theory paper but try not to panic, and stay tuned for the latest results as they are presented this week.


[1] Welcome to top physics LHCb!

[2] Christian Schwanenberger, DØ Physics co-ordinator – ad verbatim, too many times to count and in various accents.


James Howarth James Howarth is an experimental PhD student studying at the University of Manchester, UK. His physics interests centre on Top Quark Physics in general and on Top Properties in particular.

What should we know about the Higgs particle ?

1) Introduction and history

On the 4th of July, CERN announced the discovery of a new particle that can be interpreted as the Higgs boson with both the ATLAS and CMS experiments. Since this is one of the most important discoveries over the last 10 or 20 years in particle physics, let’s have a look to the full story.

First, it might be useful to look back in time. Instead of giving a list of books (there are many good ones), let me give you a few highlights quoted by Peter Higgs during one of his latest talk given in Wales on the 12th of July.

Peter Higgs

During the talk, Peter reminded us about many physicists who played a key role in the development of what has become known as the Higgs mechanism. And one would be interested to read the biographies of Yoichiro Nambu, Jeffrey Goldstone, François Englert, Robert Brout, Sheldon Glashow, Steven Weinberg, Abdus Salam, Martinus Veltman, Gerard ’t Hooft, and many others…

An important year with an amusing history was 1964, with the proposal of a new particle by R. Brout, F. Englert and Peter Higgs. On the 24th of July, the paper “Broken Symmetry Massless Particles and Gauge Fields” was accepted by the Physics Letters editor at CERN. In this paper the authors explained how to deal with the broken symmetry problem.

On July, the 31st, Peter sent his own paper “Broken Symmetries and the masses of Gauge Bosons” to the Physics Letters editor, in which he then explained how the mechanism proposed just above could (in practice) be implemented. It was rejected! During the summer, Peter reviewed his paper, adding “It is worth noting that an essential feature of the type of theory which has been described in this note is the prediction of incomplete multiplets of scalar and vector bosons”. On August 31st, the revised paper was received by Physical Review Letters, and was finally accepted! At the same time, the referee (Y. Nambu) mentioned that the same idea had just been proposed by R. Brout and F. Englert, with the paper “Broken Symmetry and the masses of Gauge Vector mesons”, which had been released exactly at the same date…

In 1975, John Ellis, Mary Gaillard and Dimitri Nanopoulos published “A phenomenological profile of the Higgs boson”. Today it could make for amusing reading: “ We apologize to experimentalists for having no idea what is the mass of the Higgs boson, …, and for not being sure of its couplings to other particles, except that they are probably all very small. For these reasons we do not want to encourage big experimental searches for the Higgs boson…”. Thankfully the scientists didn’t strictly follow these recommendations…

To close this historical part, here are some links to the original articles quoted above:

-       The paper from R. Brout and F. Englert:

-       The paper from P. Higgs:

-       The paper from J. Ellis, M. Gaillard and D. Nanopoulos:

2) Motivation

During the last century, many discoveries have been made, culminating in what we call today the Standard Model.

This Standard Model has been very successful, as it could both describe and predict some experimental measurements. For instance it predicted the  measurement of the anomalous magnetic dipole moment of the electron with an accuracy of around 1 part in 1 billion (a = 0.00115965218073 !). It also describes the components of matter and 3 out of the 4 interactions, but there are many remaining questions.

One of them is to understand differences in terms of mass scales. Why an electron (911 keV at rest) has such a different mass than a proton (938 MeV)… The same for other particles? Where does mass come from? etc… To answer these questions, many ideas have been proposed over the last decades, but only the Higgs mechanism remains as a serious option. One could think that the Higgs particle comes with a field, and that this Higgs field would be responsible for giving masses to particles.

3) Summary of all searches

3.1) LEP

At the previous main accelerator of CERN, called the LEP (, physicists started to look for the Higgs particle. The experiments ran from 1989 to 2000, and at the end, it had collected 2461 pb-1 of e-e+ collision data at centre-of-mass energies between 189 and 209 GeV. The results of the four main experiments had been merged to establish a lower bound of 114 GeV on the Higgs searches, at the 95% confidence level (C.L.)

At the end of the experiments, there was some excitement, as scientists seemed to observe an excess of 2.3 sigmas around 98 GeV, and another one of 1.7 sigma at 115 GeV ( So, part of the scientific community wanted to continue to take data with LEP, and some others thought it should be better to start the next one, say the LHC.. The latter won…

Higgs searches at LEP

3.2) Tevatron

Afterwards, the hunt for the Higgs continued at the Tevatron, Fermilab, in Chicago ( It took proton-antiproton collisions at 1.96 TeV from 1983 to Sept. 30, 2011, and the two big experiments (CDF and D0, collected about 10 fb-1 of data, and released in June 2012 the latest update. Beyond the wonderful discoveries that the Tevatron allowed (with for instance the top quark in 1995), it gave an exclusion for a Standard Model Higgs with mass between 147 and 180 GeV, and between 100 and 103 GeV, at 95% C.L. It also saw an excess with significance of 2.5 sigmas around 120 GeV.( )

Tevatron and LEP Higgs searches (2009)

3.3) LHC

3.3.1) 2011

Last year, both experiments released some promising results constraining the possible allowed region of energy of the Higgs mass, using up to 4.9 fb-1 of proton-proton collision data at sqrt(s) = 7 TeV

The Higgs boson mass ranges 112.9-115.5 GeV, 131-238 GeV and 251-466 GeV were excluded at the 95% CL, while the range 124-519 GeV was expected to be excluded in the absence of a signal. An excess of events was observed around mH ~ 126 GeV with a local significance of 3.5 sigmas.

Combined LHC and LEP Higgs searches (2011)

3.3.2) 2012

- 4 July 2012: Higgs discovery announced at CERN from both experiment ATLAS and CMS.

- ATLAS recorded about 6.3 fb-1 at 8 TeV. Two main channels have been used: H->Gamma Gamma and H->ZZ->llll. The combined analysis revealed an excess of 5 sigmas at 126.5 GeV (4.5 sigma for H-> Gamma Gamma, and 3.4 sigmas for Higgs into 4 leptons).

- CMS took about 6.3 fb-1 at 8 TeV too. It found 4.1 sigmas in H-> Gamma Gamma, 3.2 sigmas in 4 leptons, leading to a combined significance of 5 sigmas at 125 GeV. It also observed 5.1 sigmas by combining Gamma Gamma + ZZ + WW channels.

Here are the two papers accepted by Physics Letters B on the 11th of August:

ATLASObservation of a new particle in the search for the Standard Model Higgs boson with the ATLAS detector at the LHC

CMSObservation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC

ATLAS Higgs Combination Confidence plot

4) What’s next ?

Well, this is what is very exciting. This new discovery opens a large area of research. Many questions are now in the pipeline:

-       Is this new particle fully compatible with the Standard Model ?

-       What is the nature of this particle ? Is it THE Higgs boson ?

-       What is the spin ? (zero ??)

-       What are the couplings to other particles ?

-       What about the Higgs mechanism ?

To answer some of them, we need to accumulate much more data, and it is really hard to predict how much would be enough to get the next surprise… At least it is planned to collect data with the LHC till the end of 2012. We’ll see…

5) Just a last word on the Higgs decay

As we have already seen, the Higgs boson can been observed in several decay channels, such as Higgs into Gamma Gamma, Higgs into ZZ (giving 4 leptons), and also Higgs into WW. As we can see on the plot below, there are other channels we can use to detect the Higgs particle. But due to some important backgrounds (like QCD), this will require more data. After the discovery of the Higgs particle it is important to detect the Higgs decay into several decay channels to check if the coupling strength is in proportion to the mass for all fermions as the standard model predicts.


Higgs decay modes

Marc Goulette Dr. Marc Goulette is a particle physicist working on the ATLAS experiment for the University of Geneva. His research interests in Particle Physics are: Standard Model and Electroweak Physics (W, Z, electrons, photons), MC Generators, structure functions, Di-boson Physics, Higgs searches, Neutrinos, W’, Z’, new physics searches in general, and outreach activities.

A new particle is born, but who is the father?

Discovery of a 126.5 GeV Boson by ATLAS

ATLAS Higgs Combination Confidence Plot.

We have discovered a particle. It is perhaps the particle everybody has been looking for but, for now, let us just call it a particle possibly known as the Higgs.

I’m 28 years old, I have spent 4 of them in the ATLAS family and particle physics in general. That means this is the first particle discovery I have taken part in, and that is exciting even for the most level-headed of us.

Ironically the discovery made me think of how it must have been to discover the W and Z particles in UA2 and UA1 the same year I was born. Many of the slightly older members of ATLAS remember those days – I have only been told bed-time stories by my professors.
What strikes me is the magnitude of these discoveries. Did they realise these were the long-sought Weak bosons predicted by the Standard Model? Did they admit to believe it and — perhaps more interesting — When? The titles of the published articles implies a level of caution:

“Experimental observation of isolated large transverse energy electrons with associated missing energy at sqrt(s)= 540 GeV”
- The UA1 Collaboration

“Observation of single isolated electrons of high transverse momentum in events with missing transverse energy at the CERN p-pbar collider”
- The UA2 Collaboration

But, they had just discovered the W boson! Better not be too brash too early, but the titles are vague (and notice the length of the author lists)!

So, now we discovered a particle as well. Is it a Higgs, and if so, which one? Currently I’d say that this question is a bit academic, we only have one really precise theory that has been tested many times over, and that theory usually comes with one specific Higgs boson. The rest must be manifestations of a hopeful dreamer. Now I like to indulge in daydreaming, in fact that is what I’m paid to do, so let us explore what else is out there and why it is very likely that what we found is a standard model Higgs.

SUSY, Technicolor and other Dream-states

Our theory-minded friends have had plenty of time to dream up beautiful alternatives to the Standard Model. Some hope to supplant the status quo with more mathematically appealing models unifying the particle world in Grand Unification Theories (GUT). Others are going for the deeper truths combining the microcosm of particles with gravity into what are affectionately called TOE(s) or Theories of Everything. Some of these theories lean against the Standard Model’s solution for adding mass to otherwise mass-less particles. Others might use entirely different schemes. But common to even the more obvious alternatives is that they provide quantifiable(/falsifiable) predictions.

One way to tell different models apart is by looking at how the models vary in their predictions. Common (I think) to all Higgs look-a-likes is that they are unstable and decay fast in various ways. We call the probability for a specific decay the decay-mode branching-ratio. For example, the results shown from ATLAS and CMS both rely on two main decay-modes: one in which the Higgs decays into two photons and another in which the Higgs decays to two Z bosons that each again decay to either two muons or two electrons. For the standard model and supersymmetry (SUSY) some of my friends and I published a small report with the predicted branching ratios. Let’s compare:


Top: Standard Model branching ratio predictions. Bottom: SUSY MSSM predictions of a light higgs.

For a few reasons, the figures are not directly comparable. The top one shows that SUSY, even constrained to the Minimal Supersymmetric Model (MSSM), which is a small subset of all possible ways to describe Supersymmetry, still has a lot of freedom in terms of free parameters. In the plot, one of them tan(beta) is fixed at 10, but this value could be something else in reality. The other difference is that MSSM actually requires FIVE Higgs particles to give mass to both “up” and “down” type quarks, as well as charged leptons.

Luckily just the mass of one of the Higgs together with tan(beta) is enough to estimate the mass of the rest. What is relevant is that the branching ratios are different for the Standard Model Higgs and the SUSY Higgs. So, as we see our measured particle decay to two photons and two Zs, it is already in better agreement with the SM Higgs than the SUSY one in the figure.

Branching ratio for a composite Higgs in Technicolor

What can happen, will happen, but how often?
Another way to measure the difference is simply to look at the overall production cross-section, or how many times per proton-proton collision do we find a Higgs particle with any decay-mode?

In the presentations made by the two collaborations we see that the expected Standard Model Higgs cross-section is a bit less than what has been measured. But, there is nothing alarming here. The analyses are based on very few Higgs candidates, so we might just have been lucky to produce a bit more than expected statistically, and it might even out when more data is collected.

Spin it
Is it a Higgs at all? One of the most fundamental observables that separates a Standard Model Higgs from other particles is its spin, or rather its lack of spin. Measuring the spin of the Higgs particle requires a lot of statistics, hundreds of times more collisions than we have collected so far. The result from that measurement will be well worth waiting for, as the Higgs particle is the only particle in the Standard Model with zero spin, something not observed before in any elementary particle.

It might be too early to tell who the father is, but based on the baby’s hair colour I’d not be too worried about the postman yet!

Morten Dam Jorgensen Morten Dam Jørgensen is a PhD fellow at the Niels Bohr Institute in Copenhagen, Denmark. He is currently working on searches for long-lived particles and general model independent searches for deviations from the Standard Model. You can find more information at

Melbourne Dispatch: A First Coming To Terms with Discovery

Melbourne Convention and Exhibition Centre

Where to begin? The 4th of July, 2012 will remain burned in the memories of those of us fortunate to be delegates at this historic 36th International Conference on High Energy Physics (#ICHEP2012) in beautiful Melbourne, Australia. My day began in a Boeing 747, dodging tropical clouds high above the Pacific Ocean. One connection in Sydney, two shuttle buses, and one shower later, I found myself in the plenary hall at the stylish and serene Melbourne Convention and Exhibition Centre.

Projected onto the giant Melbourne screen was a live video connection to the CERN amphitheatre in Geneva, where the much awaited Higgs boson search update talks, timed for ICHEP, were about to commence. My sentiments immediately turned to Dec. 13, 2011, when I was at CERN to live-tweet the “tantalizing Higgs hints” talks for McGill. This time around, I was among a handful of physicists who’d agreed to help tweet the Melbourne conference on behalf of CERN.

Your Melbourne correspondent, with other ATLAS colleagues, including former and future spokespeople Peter Jenni and Dave Charlton. Credit: Claudia Marcelloni

The seminars that followed have already been widely discussed in blogs, newspapers, and other media, right down to the choice of font used. Through my tweets, under the handle @AWarb and hashtag #ICHEP2012, I endeavoured to convey minute-by-minute happenings while maintaining some editorial restraint. It felt a little like being a play-by-play broadcaster at a momentous sporting event, but one in which I played for one of the teams, the score was basically tied, and the teams were both separately and mutually victorious.

Minutes before the talks began, there was a distinct air of anticipation amongst the Melbourne delegates. On our giant screen, we watched as the CERN camera zoomed to one of the Geneva attendees, a placard reading “Ciao Mamma!” visible on his desk. The audience in Melbourne chuckled nervously. Then we saw Professor Higgs and the other living progenitors of the theoretical Higgs-mechanism concept entering CERN’s amphitheatre, to loud applause. Melbourne reacted as well. But when CERN’s Director-General Rolf-Dieter Heuer entered the room and prepared to initiate the proceedings, our Melbourne hall fell utterly silent.

Joe Incandela, spokesperson for the CMS collaboration, began with the first talk. Excitement at CERN and in Melbourne mounted as the results, initially articulated carefully for each of the studied decay channels, culminated in a discovery-level combination to reveal, unequivocally, that we had a new particle on our hands. The CMS team had even analyzed its data enough to be able to quote the new particle’s mass with an impressive precision of under 0.5%. Moments later, I tweeted “Day’s big question: are these Standard Model #Higgs bosons, or something else? There’s something there, but what is it exactly?

Then, it was spokesperson Fabiola Gianotti’s turn to present our ATLAS collaboration’s Higgs search results. We ATLAS collaborators already knew our outcome, but it was nevertheless thrilling to see it presented in this electrified forum. When Fabiola’s talk climaxed with our combined-channel statistical significance, the Melbourne delegates erupted in sudden loud applause followed by an immediate, eerily stunned, silence. Appreciation for the historic importance of this amazing day had now fully gripped the hall.

Meanwhile, an interesting sequence of events had occurred. CERN’s press release had been embargoed until the start of the second hour of the event, during Fabiola’s ATLAS talk. The moment it went public, my Twitter feed began to light up with scientific commentators, news organizations, and others heralding the discovery. Because of the timing, the ATLAS discovery punchline actually got sent to the rest of the planet before Dr. Gianotti reached it in her talk to the CERN and Melbourne scientific audiences.

The ICHEP meeting is now in full swing here in Melbourne. We’ve had detailed talks and discussions about the key ingredients of this new particle discovery. Other delegates and I have been tweeting updates with both Higgs and non-Higgs news. We’ve also been getting interesting questions from the worldwide public, some scientifically detailed, some more in the category of outreach. We’re gradually getting better at putting Greek and other scientific symbols into tweets! G’day mates, from Down Under.

Andreas Warburton Andreas Warburton tweets as @AWarb and is an associate professor in the Department of Physics at McGill University. For information about his research and other interests, see!/AWarb.

Very exciting day at CERN about the Higgs ??!

Good morning science addicts and everyone !

What a special day at CERN today ! Indeed, the ATLAS and CMS experiments have just released some outstanding results and observations about the search for the Higgs boson, and the ATLAS and CMS spokespersons (Fabiola Gianotti, and Joe Incandela) just presented those results in the main auditorium at 9 a.m (CERN time).

Just before we get to the big news, let me describe a little bit the atmosphere we could feel around. Knowing the seminar would be at 9 am, and the doors opening at 7:30 am, I thought that arriving at 7 am would make it (to get a sit in the main auditorium). What a fool !! Arriving at the main restaurant, the queue was already going till the new building of restaurant 1 ! Then, approaching the room, I heard that some students had the good idea to spend the whole night in front of the doors of the main auditorium. Maybe those who arrived around 5:30 am this morning could enter at the very last minute, as the last lucky persons inside. Obviously this was not a big deal for the (very special) persons invited (did you hear that Peter Higgs was there ?)…

Thankfully, there were several webcast retransmissions, for instance in the council chamber (reserved for the press), all the meeting rooms in building 40, the globe, TH, IT and TE auditoriums, as well as Prévessin. In short, pretty much everywhere we meet in CERN.  In addition, two live channels were webcast (one for CERN users, and one totally open to the public). So, as you could expect, I couldn’t get into the main auditorium, but went back to the office to watch the CERN webcast.

Discovery of a 126.5 GeV Boson by ATLAS

ATLAS Higgs Combination Confidence Plot. Note the peak at 126.5 GeV!

9 a.m. Joe started. Mmm impressive atmosphere, with a huge tension. Even Joe admitted that, joking that

he didn’t know why ;-)… Then, he started one of his 170 slides or so… saying that this was still not enough to explain, show, and thank the huge amount of work, and all the persons involved in this extraordinary adventure !

Now let me get to the main point: 9:30 – CMS has observed a new boson with a mass of 125.3 +/- 0.6 GeV with 4.9 sigma significance. Tremendous applause. We’ll come to the details a bit later…

About one hour later, Fabiola announced the remarkable results obtained by ATLAS: Combining results on various decay modes

also observe a boson of 126.5 GeV mass with a local significance of 5 sigma!! We found it ! And what a good agreement between the 2 experiments !

As the speakers said, we should thank (and think about) all the people involved in the different fields (accelerators, detectors, theory, computing, labs, personnel, engineers, technicians). We can be all proud of these great and amazing scientific achievements.

As our Director General (Rolf Heuer) said, this is a historical milestone. Fabiola also reminded us that this is also the start of the Higgs era. Some other questions are already on our mind: Is this a Standard Model Higgs ? What are its properties ? Can we find other particles ? What about the Higgs mechanism…? Such an exciting time.

As this is very interesting, I’ll try to write very soon (in the coming days) a complete article about the Higgs discovery, with more details about the decay channels, and some historical references.

Best wishes, Marc

Marc Goulette Dr. Marc Goulette is a particle physicist working on the ATLAS experiment for the University of Geneva. His research interests in Particle Physics are: Standard Model and Electroweak Physics (W, Z, electrons, photons), MC Generators, structure functions, Di-boson Physics, Higgs searches, Neutrinos, W’, Z’, new physics searches in general, and outreach activities.

Quark Excitement: Is there anything smaller?

happy particle orb

Credit: Austin Miller - The Mangusta Art Collective

The Large Hadron Collider commands many superlatives. One of the most useful of these is that the LHC is our planet’s most powerful human-built microscope. The higher the collision energy, the tinier the distances you can study.

At parties or in elevators, high-energy physicists often begin their answers to the “So what do you do?” question by mentioning the atom. From there, they work down to the nucleus, and then, if they’re lucky enough to be in a quiet party or a tall building (or a slow elevator), they get to the quarks, electrons, maybe even gluons. At this point the person asking the question is sometimes still willingly around. He or she may then even ask, “What’s inside a quark?”

The answer is that we don’t know for sure. The most successful theories of particle interactions are currently quite content with the notion that quarks are point-like particles with no spatial extent. As experimentalists, though, we’re compelled to use this magnificent new LHC microscope to probe the frontiers of minuscule distance scales empirically.

How to look inside quarks? If a quark could be broken apart, even the LHC’s energy may not be enough to pull this off. But if we could use the LHC to make a quark that’s excited, then we would know that quarks likely have substructure. As in the elevator, to get your head around this it’s handy to start by thinking in terms of atoms.

Excited atoms are all around us, for example inside light bulbs (when they’re turned on, that is). They’re more energetic than regular atoms because their electrons are in higher energy states. When the electrons return to their ground states, the excited atoms release photons of light, causing the light bulb to glow, and return to being ordinary atoms. The details about how all this works are important, but not for this blog post. What’s important here is that an atom would not be excitable if it weren’t made from smaller parts.

Anatomically incorrect quarks

Credit: anonymous

Using the ATLAS detector at the LHC, we have begun to apply this very same idea to check whether it’s appropriate to consider quarks to be point-like at these high energies. If the LHC were able to create excited quarks, we should observe them as they return to being regular quarks and emit photons of light. In a paper published in the Physical Review Letters journal, our ATLAS collaboration reported that, were a hypothetical excited quark to exist, implying quark substructure, such a state of matter most likely would not have a mass less than the equivalent of 35% of the LHC’s 2011 collision energy (which was 7 trillion electron-Volts when this data set was accumulated).

This recently published result involved a study of jets of particles, presumed to be caused by a regular quark, produced in association with photons of light. By examining the directions and energies of these jets and photons flying out from the LHC’s proton-proton collisions, we looked for evidence that they arose from the disintegration of a heavier new exotic parent particle, like an excited quark. While the search for quark substructure by way of excited quarks was one aim of this study, we note that the results can be applied generically to any hypothetical new particle that decays to a jet and a photon. These findings complement earlier and ongoing LHC searches for pairs of jets (dijets), which could also arise from quark excitations and other interesting exotic processes.

With the 2012 increase in the LHC’s energy to 8 trillion electron-Volts, and other increases in the future, we expect to continue to extend the reach in our understanding of matter’s most fundamental constituents. It’s all very exciting.

Andreas Warburton Andreas Warburton tweets as @AWarb and is an associate professor in the Department of Physics at McGill University. For information about his research and other interests, see!/AWarb.