Category Archives: Uncategorized

String Enthusiasts, Take Note(s)

It’s not every day that a group of young people strikes a chord in the world of strings. But one of the great things about New York is you’re just as likely to find them showing their stuff in a back room of a bar, or in someone’s living room, as you are in a famous hall with a big audience.

The Amphion String Quartet is making the rounds of the small spaces and hidden gems of New York City venues, but they’ve already been noticed: they’ve been selected to be among the young members (so-called CMS 2) of the famous Chamber Music Society of Lincoln Center next year, a real mark of honor. On Sunday, they gave vigorous and exciting performances of Wolf, Janacek, Gershwin and Mendelssohn in the tiny back room at barbès, a little bar in Brooklyn. Go hear them before they’re well-known and ticket prices triple!  Fortunately, they’re playing a lot over the next week and over coming months.   Physicists at Brookhaven National Lab, take note: they are playing at the lab tomorrow, Wednesday the 17th; one link says noon, another 3 pm, so I’m not sure of the actual time.

Not sure whether to trust me?  You don’t have to. Listen, or watch, for yourself.  I don’t think you’ll be disappointed.

[p.s. A trusted friend also recommends the Momenta Quartet.]

Seeking Reader Input

So I think the time is approaching for a serious overhaul of this website.  First, there’s that new particle, which very much resembles a Higgs particle, though we’re not sure if it is of the simplest type; clearly many of the older pages on the website have to change to reflect this new information.  Second, the website has grown organically and now resembles an out-of-control thicket; it is difficult to navigate and to manage.  Moving pages around on a website, with all of their cross-links, is a major challenge and not necessarily advisable; one option is to provide a map or guide of some sort, with advice about which pages are devoid of technicalities, which are a bit more advanced and suitable for anyone with freshman physics background, and which ones are rather technical.  And after July’s big success at the Large Hadron Collider, August seems like the best month to get some of this work done.

But well before I start, it’s time for me to get advice from you.  The current purpose of the website is to help you answer your questions about particle physics and related subjects, including wider questions about how science is done. I am curious to know: what are the things about the website that make it difficult for you to find what you are looking for, and what are the things that you feel might help the most?  Please, in answering, consider letting me know what your level of background knowledge is, and perhaps some insight into your goals.  This information will help me understand your suggestions in proper context.

During the overhaul period I suspect blog posts will be somewhat reduced in quantity, but I’ll keep you posted on especially important issues.   And I’ll be producing my “How the Higgs Field Works” series, as well as tying off some loose ends on a couple of other incomplete series.

A Violation of Lepton Universality?

A brief mention today of a new measurement from the BABAR experimental collaboration, which tests lepton universality (to be explained below) and finds it wanting.

The Basic Story (Slightly Oversimplified)

Within the Standard Model of particle physics (the equations that describe and predict the behavior of all of the known particles and forces), the effects of the weak nuclear force on the three leptons — the electron, the muon and the tau — are all expected to be identical. This called “lepton universality”. Continue reading

A Visit to Brandeis

Yesterday, I was visiting Brandeis University, where I gave a colloquium on the Large Hadron Collider [LHC]. It was such a gorgeous June April day outside that I felt quite lucky to see a substantial audience, which I think reflects a sense in the wider physics community that the excitement surrounding the LHC no longer reflects merely its future, but also its present.

I also enjoyed a research talk given by a Harvard postdoc, Matt Reece.  He had some things to say about which variants of supersymmetry can now largely be excluded by LHC data, both directly from existing searches for superpartner particles, and indirectly from the search for the Standard Model Higgs particle (the simplest possible form that the Higgs particle might take), assuming the hints of a Higgs particle with a mass of 125 GeV/c2 turn out to be the real deal.  He also made a nice little back-of-the-envelope calculation in his introduction, showing how that the hierarchy (the same one for which we have a hierarchy problem) between the incredible weakness of gravity and the strengths of the other forces is required for there to be large objects (i.e. planets, stars) that are held together by gravity, but prevented from collapse into a black hole by the effect of electromagnetism.

These days, at every place I visit — and Brandeis was no exception — the question of the Higgs particle always comes up.  No surprise; it’s the hottest topic in particle physics right now.  And the quality of the evidence always gets discussed.  Dr. Reece was of course asked his opinion by someone in the room.  So I get a chance to hear a lot of viewpoints.

One of the things that I have found puzzling is that almost all of the other particle physics and string theory bloggers not only are of the opinion that the Higgs particle has definitely been found, but also claim that almost all other particle physicists think so too. I honestly just can’t understand how they can say this.  I find plenty of theorists who say things like “well, if I didn’t already have a strong theoretical reason to believe the Higgs particle exists, I wouldn’t be very confident in the evidence that is in the current data.” Meanwhile, many senior experimenters regale me with stories of past errors and biases, some of which you find in the history books, and some of which you can’t. All of this is anecdotal; I can’t tell you how opinion is really distributed. But clearly a substantial fraction of the community — maybe a minority, but not a small one — are much less confident than most of the bloggers.  It’s not a question of nay-saying — I haven’t heard anyone argue there’s no evidence at all for a Higgs at 125 — but many physicists feel that the evidence is too weak at this stage for any certainty.  Of course we all expect the uncertain situation to be resolved in 2012.

One of the other great pleasures of visiting other universities is that I always get to hear about interesting research directions being pursued that I’d not been following.  (The theorists at Brandeis were all doing neat stuff that unfortunately would take way too long to describe here. ) And then, as the day comes to a close, I always hear some good stories.  My favorite this time was of an ATLAS experimentalist describing how easy it is to get lost while crawling around, installing or fixing things, inside the vast muon system of the ATLAS detector (one of the two general purpose detectors at the LHC.)   Can you imagine getting lost inside your own experiment?! 🙂  Well, this one’s the size of an eight-story office building, but hasn’t got hallways, elevators, big EXIT signs, or even an obvious THIS WAY UP.  Meanwhile, your GPS device doesn’t work down there either!

Getting (Un)Confused About Matter and Energy

In the last couple of weeks I produced an article on Mass and Energy (along with a bit on Momentum).  But I warned you in that article not to confuse “Mass” (which is somewhat related to Energy, after Einstein’s work) and “Matter” (which is not related to energy, despite the fact that the phrase “matter and energy” can be found all over the place.)

I’ve just finished a new article “Matter and Energy: A False Dichotomy”.  This article points out that “matter” is defined differently in different contexts.  And in each of those contexts, matter and energy aren’t opposites, partners, or in any other way crisply related.  The situation is inherently confusing — but I hope the article itself cuts through some of that confusion and helps clear up the matter.

At the SEARCH Workshop on the LHC

This weekend I am fortunate to be participating in a very interesting workshop on Large Hadron Collider [LHC] physics, held at the University of Maryland. Called the “SEARCH Workshop“, it was organized by three theoretical particle physicists, Raman Sundrum (University of Maryland), Patrick Meade (SUNY Stony Brook) and Michele Papucci (Lawrence Berekely Laboratory), and they’ve brought together many theoreticians and experimentalists of all stripes from within the Big Tent of LHC physics. With the exception of a panel discussion at the very end, all of the talks are experimental, from ATLAS and CMS. We’re hearing about all of the major searches that ATLAS and CMS have done at the LHC —- starting yesterday with searches for Higgs particles, heavier partners of the top quark and bottom quark, and several variants of supersymmetry — and there’s lots of time for detailed discussion.

I can’t possibly review everything being shown in the talks — ATLAS and CMS have done a huge number of analyses. But I’ll point out a couple that caught my eye that I haven’t specifically talked about in past posts (which include ones here, here, here, here, and here.) Continue reading

A Lot Going On in the Solar System

Between the Sun and the planets and the moon, there’s a lot to keep track of right now!

  • The Sun is sporting a huge dark spot large enough (but not safe unless you have a special filter or use other means) to be seen with the naked eye (learn how to view the sun safely before you try anything!!!!)
  • A huge eruption on the sun (a solar flare) a couple of days ago has made it into the mainstream press, so you’ve read all about it; its arrival at earth was a bit less powerful than expected, but there’s still enough activity going on that there’s a reasonable chance of Northern Lights and Southern Lights (i.e. auroras) even in places that don’t normally have them, such as the New York City area and other areas at similar latitudes. If you’re in northern Europe you should be looking now.
  • The planets Venus and Jupiter, the brightest objects high in the western sky after sunset, appear to be approaching each other day by day, as seen from Earth (though their actual distance from one other is much greater than the distance from the Earth to the Sun).  They’ll appear to pass by each other in just one week.  As you keep an eye on their daily progress, and see how the apparent distance between them shrinks night by night, you may enjoy considering how rapidly the planets move, as they traverse such immense distances.
  • The planet Mars rises at sunset and is a bright red dot almost due overhead at midnight, because the Earth lies almost on the straight line between Mars and the Sun. And the Moon, just past full and starting to wane, is not that far in the sky from Mars tonight — though it was much closer last night and will be further away tomorrow.  Remember it has to get all the way around the Earth every 28 days or so, so it appears to move much further every night than do the planets.

So if you’re lucky enough to have clear skies, go out and enjoy them!

Taking Stock: Where is the Higgs Search Now?

Today, we got new information at the Moriond conference on the search for the Higgs particle (in particular, Phase 1 of the search, which involves the search for the simplest possible Higgs particle, called the “Standard Model Higgs”) from the Tevatron and the Large Hadron Collider [LHC], the Tevatron’s successor.  With those results in hand, and having had a little time to mull them over, let me give you a short summary.  If you want more details, read today’s earlier post and yesterday’s preparatory post.

Before I do that, let me make a remark.  There is a big difference between healthy skepticism and political denialism.  I get the impression that some people who are writing or reading other blogs misinterpret my caution with regard to experimental results as being somehow a political and unreasonable bias against the Higgs particle being present, either at a mass of 125 GeV/c2 or at all.  That’s ridiculous.  All that is going on is that I simply am not convinced yet by the data.  I’m a careful scientist… period.  And you’ll see that I’m consistent; later in this post I will advise you not to over-react negatively to what ATLAS didn’t see.

What happened today at the Moriond conference?

What did we learn?

The Tevatron experiments see a combined 2.2 standard deviation [2.2 “sigma”] excess in their search, consistent with a Standard Model Higgs particle with a mass anywhere in the range of 115 to 135 GeV/c2.  This is not inconsistent with the Higgs hints that we saw in December from the LHC experiments.  Here I am being perhaps overly careful in not saying, more positively, “it is consistent with the Higgs hints…” only because this measurement is intrinsically too crude to allow us to narrow in on 124-126 GeV, where ATLAS and CMS see their hints.  In short, the Tevatron measurement could, in the end, turn out to indicate a Higgs at a different mass than the one indicated by the current ATLAS and CMS hints.  Anyway, it’s a minor and mostly a semantic point.

The results from ATLAS were a bit of a shock.  In all three processes on which ATLAS reported, CMS has presented results already, and in each case CMS saw a small excess (1 standard deviation [1″sigma”], which is  small indeed.)  But ATLAS reported today that it sees essentially no excess in any of the three, and even a deficit in one of them for low mass.  This has a big effect.

  • First, it allows ATLAS to exclude a Standard Model Higgs all the way up to 122 GeV/c2 (except for a little window 1 GeV/c2 wide centered at 118) and down to 129 GeV/c2.  The only large window left for the Standard Model Higgs particle is 122-129, more or less centered around the hint at 126 GeV/c2 that they saw in December.
  • But second, the significance of the December hint, when combined with the new data that shows no excesses in these three new processes, drops by about a full standard deviation.  That’s a pretty big drop.

What does it all mean?

I think it basically means, roughly, status quo.  We got some positive information and some negative information today, and none of it is that easy to interpret.  So I think we are roughly where we were before, except that we probably no longer have to worry about any Standard Model Higgs below 122 GeV/c2.  Before today we had a decent hint of a Standard Model-like Higgs particle with a mass around 125 GeV/c2; we still have it.  Let me explain what I mean.

There are easy (relatively!) searches for the Higgs, and there are hard ones.  The easy searches are the ones where the backgrounds are relatively simple and the signal is a narrow peak on a plot.  There are two:

  1. Higgs decaying to  photons
  2. Higgs decaying to two lepton/anti-lepton pairs (often called “four leptons” for short)

Results on these were presented by both ATLAS and CMS back in December.  The hard searches are the ones where the backgrounds are rather complicated and the signal is quite broad, so that a mistake in estimating a background can either create a fake signal or hide a real one.    There are three (mainly) for a lightweight Higgs:

  1. Higgs decaying to a lepton, an anti-lepton, a neutrino and an anti-neutrino
  2. Higgs decaying to a tau lepton/anti-lepton pair
  3. Higgs decaying to a bottom quark/anti-quark pair

These are the three that ATLAS reported on today (where they saw no sign of a Higgs signal), and that CMS presented back in December (and saw a small excess in all three.)  [ATLAS presented a result on the first one in December, but only using part of their data; it showed a small excess at the time, but not now.]  The third process is the main one in which CDF and DZero reported an excess today, though the first one also plays a role in interpreting that excess.

In other words, everything we learned today had to do with the difficult searches — the ones that are hard to perform, hard to interpret, and hard to check.  And everything we learned was 1 or 2 sigma information; not very compelling even statistically.

For this reason,

  • I would not conclude that the new Tevatron results make the 125 GeV Higgs case much stronger
  • I would not conclude that the new ATLAS results make the 125 GeV Higgs case much weaker

For the same reason, when I explained why I was skeptical of the evidence back in December, I told you that in my view the CMS excesses in the difficult searches did not make the case for a 125 GeV Higgs much more compelling.  Since the easy searches at CMS do not show as large excesses as ATLAS’s do, I wasn’t really comfortable with the whole case from CMS.   Their case improved in January, when they added a bit more information from their easy search for two photons.

If, like me, you discount the difficult Higgs searches somewhat relative to the easy Higgs ones, then almost nothing has changed, as far as the current Higgs hints, after today’s up and down information.  The excess in the two easy searches at ATLAS is still there, and there are excesses at CMS at least in the two-photon search.  Even from the beginning, I gave you good reasons to think the ATLAS’s easy-search excesses were a bit larger than they should be, probably due to an upward statistical fluctuation in the background.    Conversely I think now that one should not overstate how bad today’s ATLAS news is for the Higgs hints.  It’s still quite reasonable to think there may be a Standard Model Higgs there at 125 GeV/c2.  There’s some evidence in its favor, and it’s certainly not ruled out at this point. (Whereas now, almost all other masses are.)

So as usual I advise patience and calm and no hyperventilating; the 2012 data will settle the issue.  Either there is a Standard Model Higgs with a mass within a few percent of 125 GeV/c2 , or we’ll soon be fanning out in Phase 2 of the Higgs search, looking for all the other types of Higgs particles that might be out there.