CMS and ATLAS present their results

CMS results are being presented by Jim Olsen of Princeton University.

CMS has magnet problems this year due to cooling system problems but was able to record 3/4 of the data with the magnet on.

The diboson excess widely discussed this summer is, perhaps not surprisingly, not confirmed.  Same for the old dilepton excesses.

With certain assumptions, limits on gluinos jump from 1.3 TeV – 1.4 TeV to 1.6-1.7 TeV.

Big improvement in limits on “Black Holes” or anything else dramatic at very high energy (as we saw also in my post yesterday about ATLAS multijet events.)

Top-primes — limits jump to about 950 GeV relative to 800, again with assumptions.

Some new limits on invisible particles.  W’ resonances ruled out up to 4.2 TeV if they decay to leptons, to 2.4 TeV if they decay to top quark + bottom antiquark (with assumptions.)  No dijet bumps or other unusual dijet behavior.  No dilepton bumps up to 2.5 – 3.1 TeV for simple assumptions.

Diphotons (with 2.6 inverse fb of data)! (Olsen shows an event at 745 GeV).  All diphoton events used.  Peak?  Yes!!  BUT: local 2.6 standard deviations, and with the look elsewhere effect, only 1.2 standard deviations. Not impressive.   Such a peak is not inconsistent with previous results, but doesn’t look like a signal.  Still… combining old and new data we see a signal at 3 standard deviations local, 1.7 standard deviations globally after look elsewhere effect.

Also the peak is rather ragged, though this doesn’t imply anything in particular; it is worth noting.  If you assume the peak comes from a wider bump, the significance goes down.

Now on to ATLAS, with results presented by Marumi Kado (from the French Laboratoire de l’Accelerateur Lineaire and Orsay).

ATLAS has 1.2-1.5 times more useable data than CMS.  This could be important.

Look for Higgs in four leptons.  Big statistical fluke!  They see fewer events than expected! This is, of course, no big deal… if you expect 6 events it is no surprise if you happen to see 2.

No peak in two Z’s at higher mass (i.e. no heavy Higgs seen.)  Some improvement in searches for Heavy Higgs particles decaying to taus at higher mass.

Limits on gluinos (with assumptions) go from 1.2-1.4 TeV to 1.4-1.8 TeV. (Got an improvement by looking for boosted top quarks in the case where gluinos decay to top quarks.)  Bottom squarks (with assumptions) — limits go from 650 GeV to 850 GeV.

The excess in Z + jets + invisible particles in high energy events remains in Run 2, a little smaller than in Run 1 but still there.  [Run 1: 10 expected, 29 observed; Run 2: 10 expected, 21 observed.] CMS still doesn’t see it.  What’s the story here?

Dijets (as I wrote about yesterday.)  Kado shows the highest-energy dijet event ever observed by humans.  Nothing unusual in photon + jet. Nothing in dileptons — limits on typical Z’ bosons in the 3-3.4 TeV range, W’ decaying to leptons limited up to 4.1 TeV,

DIPHOTONS. Here we go.

A completely generic search for photon pairs; nothing special or unusual.  Looking for bump with narrow width up to large width.  3.6 standard deviations local, global significance is 1.9 standard deviations.  Looks amusingly similar to the first hint of a Higgs bump from four years ago!  Large width preferred, as much as 45 GeV. Local significance goes up to 3.9 standard deviations, 2.3 after look elsewhere.  Mass about 750 GeV.  Hmm.  No indication as to why they should have been more efficient than in Run 1, or why such an excess wouldn’t have been seen at Run 1.

WW or ZW or ZZ where there was an excess in Run 1.  As with CMS, no excess seen in Run 2.  WH,ZH: Nothing unusual.

Ok, now for the questions. The diphoton bump seen, with moderate significance in ATLAS and low significance at CMS, is very interesting, but without more information and more thought and discussion, it’s premature to say anything definitive.

Kado says: Run 1 two-photon data was reanalyzed by ATLAS and it is compatible with the Run 2 bump for large width at 1.4 standard deviations, less compatible for narrow width at more than 2 standard deviations.  They have not combined Run 1 and Run 2 data yet.

Kado says: the diphoton excess events look like the background, with no sign of extra energetic jets, invisible particles, etc; nothing that indicates a signal with widely different properties sitting over the standard two-photon background.  (Obviously — if it had been otherwise they could have used this to reduce background and claim something more significant.)  There are about 40 events in the peak region (but how wide is he taking it to be?) Olsen: CMS has 10 events in the same region, too little to say much.

Conclusion?  The Standard Model isn’t dead yet… but we need to watch this closely… or think of another question.

 

 

 

Exciting Day Ahead at LHC

At CERN, the laboratory that hosts the Large Hadron Collider [LHC]. Four years ago, almost to the day. Fabiola Gianotti, spokesperson for the ATLAS experiment, delivered the first talk in a presentation on 2011 LHC data. Speaking to the assembled scientists and dignitaries, she presented the message that energized the physics community: a little bump had shown up on a plot. Continue reading

First Big Results from LHC at 13 TeV

A few weeks ago, the Large Hadron Collider [LHC] ended its 2015 data taking of 13 TeV proton-proton collisions.  This month we’re getting our first look at the data.

Already the ATLAS experiment has put out two results which are a significant and impressive contribution to human knowledge.  CMS has one as well (sorry to have overlooked it the first time, but it isn’t posted on the usual Twiki page for some reason.) Continue reading

Moon Covers Venus Shortly

The Moon will occult (i.e. move in front of and eclipse) the planet Venus today, as visible (yes, in daytime, if you have binoculars or a telescope) across the United States sometime between 11 and 12:45 this morning, depending on where you live.  Earlier out west, later in the east. If you want to see the heavens are really in motion, here’s a chance.  Below is a link to an article that gives the details:

http://www.skyandtelescope.com/observing/moon-flys-by-catalina-occults-venus-on-dec-7th120220150212/

An Overdue Update

A number of people have asked why the blog has been quiet. To make a long story short, my two-year Harvard visit came to an end, and my grant proposals were turned down. No other options showed up except for a six-week fellowship at the Galileo Institute (thanks to the Simons Foundation), which ended last month.  So I am now employed outside of science, although I maintain a loose affiliation with Harvard as an “Associate of the Physics Department” (thanks to Professor Matt Schwartz and his theorist colleagues).

Context: U.S. government cuts to theoretical high-energy physics groups have been 25% to 50% in the last couple of years. (Despite news articles suggesting otherwise, billionaires have not made up for the cuts; and most donations have gone to string theory, not particle physics.) Spare resources are almost impossible to find. The situation is much better in certain other countries, but personal considerations keep me in this one.

News from the Large Hadron Collider (LHC) this year, meanwhile, is optimistic though not without worries. The collider itself operated well despite some hiccups, and things look very good for next year, when the increased energy and high collision rate will make the opportunities for discoveries the greatest since 2011. However, success depends upon the CMS experimenters and their CERN lab support fixing some significant technical problems afflicting the CMS detector and causing it to misbehave some fraction of the time. The ATLAS detector is working more or less fine (as is LHCb, as far as I know), but the LHC can’t run at all while any one of the experimental detectors is open for repairs. Let’s hope these problems can be solved quickly and the 2016 run won’t be much delayed.

There’s a lot more to say about other areas of the field (gravitational waves, neutrinos, etc.) but other bloggers will have to tell those tales. I’ll keep the website on-line, and will probably write some posts if something big happens. And meanwhile I am slowly writing a book about particle physics for non-experts. I might post some draft sections on this website as they are written, and I hope you’ll see the book in print sometime in the next few years.

LHC Starts Collisions; and a Radio Interview Tonight

In the long and careful process of restarting the Large Hadron Collider [LHC] after its two-year nap for upgrades and repairs, another milestone has been reached: protons have once again collided inside the LHC’s experimental detectors (named ATLAS, CMS, LHCb and ALICE). This is good news, but don’t get excited yet. It’s just one small step. These are collisions at the lowest energy at which the LHC operates (450 GeV per proton, to be compared with the 4000 GeV per proton in 2012 and the 6500 GeV per proton they’ve already achieved in the last month, though in non-colliding beams.) Also the number of protons in the beams, and the number of collisions per second, is still very, very small compared to what will be needed. So discoveries are not imminent!  Yesterday’s milestone was just one of the many little tests that are made to assure that the LHC is properly set up and ready for the first full-energy collisions, which should start in about a month.

But since full-energy collisions are on the horizon, why not listen to a radio show about what the LHC will be doing after its restart is complete? Today (Wednesday May 6th), Virtually Speaking Science, on which I have appeared a couple of times before, will run a program at 5 pm Pacific time (8 pm Eastern). Science writer Alan Boyle will be interviewing me about the LHC’s plans for the next few months and the coming years. You can listen live, or listen later once they post it.  Here’s the link for the program.

Completed Final Section of Article on Dark Matter and LHC

As promised, I’ve completed the third section, as well as a short addendum to the second section, of my article on how experimenters at the Large Hadron Collider [LHC] can try to discover dark matter particles.   The article is here; if you’ve already read what I wrote as of last Wednesday, you can pick up where you left off by clicking here.

Meanwhile, in the last week there were several dark-matter related stories that hit the press.

There has been a map made by the Dark Energy Survey of dark matter’s location across a swathe of the universe, based on the assumption that weak signals of gravitational lensing (bending of light by gravity) that cannot be explained by observed stars and dust is due to dark matter.  This will be useful down the line as we test simulations of the universe such as the one I referred you to on Wednesday.

There’s been a claim that dark matter interacts with itself, which got a lot of billing in the BBC; however one should be extremely cautious with this one, and the BBC editor should have put the word “perhaps” in the headline! It’s certainly possible that dark matter interacts with itself much more strongly than it interacts with ordinary matter, and many scientists (including myself) have considered this possibility over the years.  However, the claim reported by the BBC is considered somewhat dubious even by the authors of the study, because the little group of four galaxies they are studying is complicated and has to be modeled carefully.  The effect they observed may well be due to ordinary astrophysical effects, and in any case it is less than 3 Standard Deviations away from zero, which makes it more a hint than evidence.  We will need many more examples, or a far more compelling one, before anyone will get too excited about this.

Finally, the AMS experiment (whose early results I reported on here; you can find their September update here) has released some new results, but not yet in papers, so there’s limited information.  The most important result is the one whose details will apparently take longest to come out: this is the discovery (see the figure below) that the ratio of anti-protons to protons in cosmic rays of energies above 100 GeV is not decreasing as was expected. (Note this is a real discovery by AMS alone — in contrast the excess positron-to-electron ratio at similar energies, which was discovered by PAMELA and confirmed by AMS.)  The only problem is that they’ve made the discovery seem very exciting and dramatic by comparing their work to expectations from a model that is out of date and that no one seems to believe.  This model (the brown swathe in the Figure below) tries to predict how high-energy anti-protons are produced (“secondary production”) from even higher energy protons in cosmic rays.  Newer versions of this models are apparently significantly higher than the brown curve. Moreover, some scientists claim also that the uncertainty band (the width of the brown curve) on these types of models is wider than shown in the Figure.  At best, the modeling needs a lot more study before we can say that this discovery is really in stark conflict with expectations.  So stay tuned, but again, this is not yet something that in which one can have confidence.  The experts will be busy.

Figure 1. Antiproton to proton ratio measured by AMS. As seen, the measured ratio cannot be explained by existing models of secondary production.

Figure 1. Antiproton to proton ratio (red data points, with uncertainties given by vertical bars) as measured by AMS. AMS claims that the measured ratio cannot be explained by existing models of secondary production, but the model shown (brown swathe, with uncertainties given by the width of the swathe) is an old one; newer ones lie closer to the data. Also, the uncertainties in the models are probably larger than shown. Whether this is a true discrepancy with expectations is now a matter of healthy debate among the experts.

Science Festival About to Start in Cambridge, MA

It’s a busy time here in Cambridge, Massachusetts, as the US’s oldest urban Science Festival opens tomorrow for its 2015 edition.  It has been 100 years since Einstein wrote his equations for gravity, known as his Theory of General Relativity, and so this year a significant part of the festival involves Celebrating Einstein.  The festival kicks off tomorrow with a panel discussion of Einstein and his legacy near Harvard University — and I hope some of you can go!   Here are more details:

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First Parish in Cambridge, 1446 Massachusetts Avenue, Harvard Square, Cambridge
Friday, April 17; 7:30pm-9:30pm

Officially kicking off the Cambridge Science Festival, four influential physicists will sit down to discuss how Einstein’s work shaped the world we live in today and where his influence will continue to push the frontiers of science in the future!

Our esteemed panelists include:
Lisa Randall | Professor of Physics, Harvard University
Priyamvada Natarajan | Professor of Astronomy & Physics, Yale University
Clifford Will | Professor of Physics, University of Florida
Peter Galison | Professor of History of Science, Harvard University
David Kaiser | Professor of the History of Science, MIT

Cost: $10 per person, $5 per student, Tickets available now at https://speakingofeinstein.eventbrite.com