Sunday, October 09, 2011

Tenson Within OPERA

I heard about this uneasiness within the OPERA collaboration since the news broke, but I didn't think it was appropriate to pass on rumors such as that. But now, with the news being reported on PhysicsWorld, it is now official that there are large section of the collaboration that want to make sure things are double-checked and looked into again before even submitting it for peer-reviewed publication.

The announcement made headlines around the world, since it appears to contradict Einstein's special theory of relativity. However, not everyone within OPERA was happy to release the results publicly, with several of the 30 group leaders within the 160-strong collaboration being opposed to the release of a paper on the arXiv preprint server and the accompanying seminars and press release without further tests of possible systematic errors being carried out. Now, a larger fraction of the group leaders is concerned about the paper being submitted to a research journal. One member of OPERA, who does not wish to be named, says there is a "lot of tension" within the collaboration and that up to half of the members are opposed to an immediate submission.
This article pointed out two possible issues with the OPERA analysis, and these are the two issues that have bothered many who have read the preprint.

One such check regards the timing of the neutrinos' arrival at Gran Sasso, and involves carrying out an analysis of timing data collected by monitoring the charge, rather than the light, generated by particles passing through the detector. This analysis relies on a very precise and painstaking measurement of the length of the cabling used to collect the timing data, in order to isolate any systematic errors that may be present within the electronics or other parts of the timing system.

Another independent check involves the statistical analysis of the data collected by OPERA. The researchers are not able to track, and therefore time, individual neutrinos as they travel from Geneva to Gran Sasso, but instead they measure the temporal distribution of the protons within each bunch just before the protons hit the graphite target and then compare this with the distribution of the corresponding neutrinos as they are detected in OPERA – with the temporal offset between the two revealing the time of flight. Some members of the collaboration argue that this offsetting procedure needs to be carried out independently, in order to be sure that the temporal profile of the neutrinos leaving CERN can be inferred accurately from that of the protons that produced them.
That last part could be crucial. They were using the proton temporal distribution to be the same as the neutrino temporal distribution. Unlike MINOS, OPERA has no Near Detector to verify the neutrino temporal distribution before they travel all that distance to Gran Sasso. MINOS, on the other hand, has a near detector right at Fermilab, with the Far Detector located hundreds of miles away in Soudan, Minnesota. In some sense, I think it will be up to MINOS, and to some extent, T2K, to verify this result, which they will be able to do within a year.

So sit tight. The next few months will be very interesting.



Anonymous said...

Hello ZapperZ: once again, how does one define the time of flight of a Dirac particle?


Confused in Corvallis.

ZapperZ said...

I don't get it. Why is this an issue? I mean, how do you define time of flight of the electrons circling the storage ring in a synchrotron center?


Anonymous said...

As Pauli pointed out way back, time is not an Hermitian operator in either Schroedinger's or Dirac's quantum mechanics so how does one measure travel time (or tunneling time) for a Schroedinger or Dirac particle?

As for the storage ring, I suppose energy (which is Hermitian and so measurable) is converted into a velocity etc., but the subsequent deduction of a circulation time is a classical albeit relativistic calculation.

Now, Sudarshan showed that special relativity does admit tachyons so observing them as classical relativistic particles would overturn nothing.



ZapperZ said...

You are kidding, right?

I mean, you can't be THAT oblivious to what we can do already. How do you think people design particle accelerators, especially in timing when the exact time the particle enters a particular section or accelerating structures?

This has nothing to do with "tunneling time", because it isn't tunneling. These are "classical" free particles. Look at beam physics codes such as PARMELA that are used to model charge particle beam dynamics in accelerator physics.


Anonymous said...

I understand that tunneling is not specifically involved here, I merely cited it as another instance of time `measurements' being problematic in QM.

OK, if it is the case that OPERA etc are modeling particles classically, then observing tachyons would be very cool but would not overturn special relativity as Sudarshan made clear.

Anonymous said...

Dear Anthony: (I'm guessing you're the author of QFT in a Nutshell, which I enjoy), it comes back to this (which I don't expect you to post).

One of the popular press accounts of OPERA reported that `some' at CERN/OPERA believe that the existence of tachyons would imply `time travel' which `others' there believe to be `impossible'. Thus we can be sure that the CERN/OPERA people are aware of Sudarshan's analysis. He showed that an observer moving with e.g. an OPERA tachyon would find them both in Italy before Switzerland.

Does this mean that the `others' reject the analysis by itself, or else have misgivings for external reasons? The analysis is so simple that it cannot be internally incorrect (and it isn't); indeed, it prompted several reputable groups to make searches for tachyons. They were unrewarded as we all know.

As I asked in my earlier comment on your earlier OPERA story, what is the issue? We all agree that a subluminal particle cannot be accelerated to go faster than light. Is it that a CERN event may have generated a tachyon `from scratch'?


ZapperZ said...

I think you've gone over the deep end.

I am not this "Anthony" guy. I also don't know why you are OBSESSING over this Sudarshan analysis.

At what point do you say "OK, I'm going to wait for other independent verifications before I spend WAAAAY too much time on this"?


Pi-Guy said...


If you pay attention over time you'll see that, whoever ZapperZ is outside of this blog, he surely isn't a field theorist living in California.