The Higgs Boson: Searching for the God Particle (34 page)

Heather Gray: A Diary of the Higgs

By Kelly Oakes

Heather Gray, a researcher working on the ATLAS experiment at CERN, was at this year’s
Lindau meeting. I spoke to her over email before it started to find out about her
expectations, and afterwards she told me about her impressions of the meeting and what it
was like to watch the announcement from CERN with other young researchers. She also
made a video diary of the Higgs exciement at Lindau that you can watch below.

What were your first impressions of the meeting when you arrived?

My arrival at the meeting was somewhat chaotic as my bag had been sent on the wrong flight by the airline and then when dashing out
for some new clothes I dropped my wallet! However the staff at the Lindau meeting where wonderful so they quickly helped me to sort
things out. This did mean that I was a little distracted during the initial stages and the opening ceremony. I enjoyed the formality and
tradition clearly inherent in the opening ceremony and obtained a better understanding of the history of the Nobel meeting. Of course,
it was exciting to see so many Nobel Laureates in one place at the same time, although this was something I got more accustomed to
over the week. Finally, I found the number of young researchers quite surprising – there really were many, many young people all doing
fascinating research in many different corners of the globe.

You mentioned in our last Q&A that you’re working on the Higgs boson. How busy were you in the weeks leading up
to the meeting, and how did you feel on Wednesday when the results were announced?

Oh, whenever we have data, we are always extremely busy working to understand it. I was very busy the week before I left because I was
taking a shift as run manager: essentially looking after our experiment’s data-taking for a week, in the last few days that we collected
data that was used for this result, so it was definitely a time in which we wanted things to go smoothly. I do work on the Higgs, but I
don’t work on the two channels that ATLAS showed this time, so for me the busiest time is yet to come as we collect more data.
However, in the last few days before the result went public the entire collaboration was involved in the process of understanding the
results, approving it and converging on the message we wanted to present to the outside world.

At Lindau, the seminar in which the spokespeople of the two experiments presented the new results unfortunately clashed with some of
the scheduled talks, so there wasn’t an opportunity to watch it together with the other participants. However, quite a group of us got
together and watched in the tent on a few laptops, which I enjoyed a lot. I found the seminar very exciting, even if I already knew what
ATLAS was going to show, and I even found myself surprisingly emotional when the Director General said “I think we’ve got it!” We
couldn’t stop ourselves from clapping as everyone had broad grins on their faces.

What’s next for the Higgs in general, and for your own research?

In this case, the two are really quite well aligned. What we want and need to do next re more careful measurements to determine
whether this particle is the Higgs boson or something else. The decay channel that I work on directly is the decay to a pair of bottom
quarks and we need data before the channel becomes sensitive. Of course, this doesn’t mean we’re waiting, but rather actively working
on the analysis and optimising things so that we can obtain the best results possible.

In order to understand if it is the Higgs, we need to repeat the current measurements with more data and check nothing changes. Then
we need to see if we observe this particle in all predicted decay channels and whether we see it decaying in different ways at the rate at
which we expect. Here’s where the channel I work on fits in, as the bb channel is the only decay to quarks that we could expect to see,
and if it’s the Higgs, it should really couple to all particles. We’ll be taking data until the end of this year or early next year and we hope
that this should give us enough data to obtain at least the first answer to this question.

You took part in a master class with David Gross – what was that like?

The master class was one of my favourite parts of the meeting. I couldn’t have asked for better timing to be giving a talk about the Higgs
than the day after that seminar! I found the contributions from all participants were of a very high quality such that I found them
fascinating. The discussion was excellent with many people attending the class participating. I also enjoyed the discussion I had with
David Gross about how sure we experimentalists need to be about a result before making it public.

What will you take away from the meeting?

I will take away some great experiences, great friends, new ideas and most of all the impressions I got of the Nobel Laureates, not just as
great scientists, but also human beings.

-Originally published: Scientific American online, July 11, 2012

Beyond Higgs: On Supersymmetry (or Lack Thereof)

By Glenn Starkman

With the search for the Higgs
boson, the last missing piece of
the Standard Model of particle
physics, apparently reaching its
long-anticipated-and-finally-successful
conclusion, anticipation
of the next set of discoveries is
growing.

Recently the Stanford campus
hosted a smallish gathering
celebrating the 60th birthday of
Savas Dimopoulos, justly acclaimed by each of the attendees as the (or at least
one of the few) most insightful particle physics model builders of the last 30
years. (And my PhD adviser.) Now you’d think that the leading topic of
discussion at such an event would be the details of the ongoing Higgs search –
has it or hasn’t it been discovered? Does the fact that the two relevant
experiments at CERN’s Large Hadron Collider (LHC) – ATLAS and CMS – both have a signal indicative of a new particle with the same
mass? And what about the supportive analysis coming from Fermilab’s Tevatron?

Surprisingly (to the outsider) this was all considered old news. Repeatedly, the theorists joked that, with the exception of the actual
CERN experimentalists present, all of us know that the Higgs has now been discovered with a mass of 125 GeV/c
²
. (It hasn’t, quite, but
the hints are strong.) The message was clear: “We’ve known for decades that the Higgs is going to be found. So break open the
champagne and get the celebrating over with, because what we really want to know is — which is the correct version of Beyond the
Standard Model physics?” With a brief nod to large extra dimensions (a Dimopoulos, and associates A, D and also I, idea) and a fond
farewell to Technicolor (another idea that Dimopoulos helped advance), the focus turned again and again to the likely suite of
Supersymmetric (SUSY) particles (yet another stock in which Dimopoulos is heavily invested).

Supersymmetry – a theory that posits that for every known particle there is another (or more than one) yet-to-be-discovered partner
particle – is the leading candidate for physics Beyond the Standard Model. It is central to string theory (a.k.a. super-string theory),
required for gauge coupling unification (see below), useful for solving the Higgs Fine Tuning Problem (definitely see below) and also
gives us the leading candidate for dark matter – the Lightest Supersymmetric Particle (LSP).

But I’m getting way ahead of myself, and probably you. Especially since I and my colleagues have come to believe that the principal
indictment of the Standard Model, which has been used to argue so forcefully for Beyond the Standard Model (BSM) physics is, hmmm,
dubious. Or as one of those colleagues would say – completely wrong. A main rationale for supersymmetry evaporates on closer
inspection.

So what is Beyond The Standard Model (BSM) physics, why are people so convinced it is around the corner, and should they be?

At least since the discovery of the W and Z particles at CERN in 1983, physicists have been pretty much convinced that the Standard
Model (SM) that emerged from the late 1960s and early 1970s is the correct model of fundamental physics. At least at energies below
the so-called weak-scale – a few hundred GeV – or maybe a few times that. But particle theorists variously hoped/expected/knew that
at higher energies the Standard Model was not the whole story, and a more fundamental theory would need to be found.

There are two types of reasons to doubt the completeness of the Standard Model – aesthetic (philosophical) and mathematical.

Aesthetic problem number one, physicists adore simplicity. Zero and one are our favorite numbers. Two can be suffered. After two
comes “too many”, although identical copies (twins, triplets, …) may receive special dispensation. The Standard Model has too many
too-many’s: three fundamental forces (a.k.a. gauge groups); way too many fundamental fermions (particles that make up matter)– three
generations each with at least 5 representations (groups) of them — plus three sets of gauge bosons and the set of particles of which the
Higgs boson is a member. It also has far too many (more than 20) independent parameters.

Aesthetic problem number two –– for no apparent reason the weak scale is much (as in about 10
¹⁶
times) smaller than what we believe
to be the fundamental energy scale of physics – the Planck scale (about 10
¹⁹
GeV), a scale set by the strength of gravity (the one
fundamental force not included in the Standard Model). This is known as the (Weak) Hierarchy Problem – and can also be understood
in terms of the absolutely enormous strength of the three Standard Model forces compared to that of gravity between pairs of
fundamental particles separated by appropriately microscopic scales.

It is however the technical problem that has carried the most weight in convincing people that there must be physics beyond the
Standard Model. It is the story we tell our children — quantum mechanics makes the Standard Model unstable. Quantum mechanics
teaches us that, as a particle such as a Higgs boson travels along, it can emit and reabsorb another particle. This process represents a
“loop contribution” to the mass of the Higgs boson, so-called because a pictorial representation of the process – Feynman diagrams
– depicts these processes as loops attached to the traveling Higgs boson.

Unfortunately, when you add up the loop contributions to the mass of the Higgs boson from all possible particles with all possible
energies and momenta, they appear to be infinite or at least proportional to the maximum possible momentum that can be carried. For
technical reasons these are called quadratic divergences and are widely derided. For the actual Higgs boson mass to be finite, there
must apparently be subtle and precise cancellation of the loop contributions against the underlying “tree” (loop-free) mass. This Higgs
Fine-Tuning Problem, so the lore tells us, must be remedied.

BSM physics is the proposed remedy. Supersymmetry cancels the loop of every known particle against the loop of an as-yet-to-be-discovered
partner particle. Technicolor eliminates the Higgs boson – replacing it by a composite of new particles called techni-quarks.
If there are large extra dimensions then the largest momentum that can circulate in a loop is actually only a little larger than the weak
scale. Clearly BSM physics is not just desirable but essential.

Recently, however, my colleague Bryan Lynn suggested, and together with Katie Freese and Dmitry Podolsky, he and I explained, how
the Standard Model actually comes up with a remedy all on its own.

The Higgs boson is one member of a set of quadruplets in the Standard Model. At energies below the weak scale, its three siblings get
eaten – they get incorporated into the W and Z bosons. According to a famous theorem due to MIT’s Jeffrey Goldstone (hence
“Goldstone’s Theorem”), the masses of the three siblings must be exactly zero. In particular, the quadratically divergent contribution to
their masses are zero.

Although this doesn’t force the mass of the Higgs boson to be zero (a good thing, since it seems likely to be about 125 GeV/c
²
), it does
mean that the quadratic divergences in the Higgs mass that have worried us for decades are not a problem of the Standard Model after
all.

Now, not everybody buys our argument. Some of them prefer to focus on the aesthetic challenge of the Weak Hierarchy Problem, while
others argue that we have no choice but to add quantum gravity to the Standard Model, inevitably resurrecting the Higgs Fine Tuning
Problem.

We would counter that the absence of a Higgs Fine Tuning Problem in the Standard Model is such a virtue that, absent any hard
evidence for BSM physics, preserving the Standard Model’s Goldstone miracle should be taken as a requirement of any proposed BSM
theories.

The implication is clear. If there is no problem, there may be no need for a solution. Beyond the Standard Model Physics isn’t ruled out
by the absence of a Higgs Fine Tuning Problem in the Standard Model, but it does mean that the Standard Model may well be the whole
story, or at least the whole story at the energies that the LHC can command. In short, don’t be surprised if the Higgs is the last new
particle discovered by the LHC. Theorists may hunger for physics beyond the Standard Model, but nature may be quite content
without it, thank you very much.

-Originally published: Scientific American online, June 20, 2012