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 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 4^th 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 12^th of July.

http://www.swansea.ac.uk/media-centre/livestreaming/higgs-boson/

http://indico.cern.ch/getFile.py/access?resId=0&materialId=slides&confId=191133

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 24^th 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 31^st, 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 31^st, 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:

http://prl.aps.org/pdf/PRL/v13/i9/p321_1

-       The paper from P. Higgs:

http://prl.aps.org/pdf/PRL/v13/i16/p508_1

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

http://arxiv.org/pdf/1201.6045.pd

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 (http://public.web.cern.ch/public/en/research/LEPExp-en.html), 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.)

http://lephiggs.web.cern.ch/LEPHIGGS/papers/LEP-SM-HIGGS-PAPER/paper.ps

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 (http://arxiv.org/pdf/hep-ph/0502075v2.pdf). 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 (http://www.fnal.gov/pub/tevatron/). It took proton-antiproton collisions at 1.96 TeV from 1983 to Sept. 30, 2011, and the two big experiments (CDF and D0, http://tevnphwg.fnal.gov/) 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.( http://arxiv.org/abs/1207.0449 )

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

http://cdsweb.cern.ch/record/1421964/files/science.pdf

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 m_H ~ 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 11^th of August:

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

CMS: Observation 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

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?

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 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 http://mdj.dk