Ice-Cold Neutrinos

This blog reveals a connection between the Freiburg-Madison-Gesellschaft and CERN, and its personal too.

©THE CAP TIMES

Francis Halzen, a Belgian-born physics professor at the University of Wisconsin-Madison and, from 1969 to 1971, a colleague at CERN*, won this year’s Nobel Prize for his neutrino research. He learned about it while sitting in a hotel room in Italy: “I got the call at around 11 o’clock. … If I had been in Madison, they would have woken me up. I slept very well last night.”

*I joined CERN in 1968 and never met him

While writing a research proposal for a committee meeting with his peers, the newborn primus inter pares said, “I hope I don’t disturb the peace there too much.”

He told Madison’s THE CAP TIMES: “This project was challenging enough that it ending up with the Nobel Prize was so unlikely that until we started to finally achieve some results recently in the last 10 years, I certainly … never thought about this, and this certainly was not a motivation.”

“I was lucky enough that the right people showed up at the right time with the right talents to overcome the challenges the project faced. Some were overcome with pure luck, but that's science,” Halzen chuckled. “It's a combination of perseverance and talent and luck.”

Neutrino research was a field in high-energy physics where CERN led when Red Baron worked there.

And CERN continued its neutrino research beyond my retirement. In 2011, experimental results caused an outcry in physics when physicists measured neutrinos produced at CERN in the Laboratori Nazionali del Gran Sasso buried deep below the mountains that were “speedier” than the speed of light.

Laboratori Nazionali del Gran Sasso (©Alastair Philip Wiper)

The OPERA* experiment, designed to study the transformation (oscillation) of muon neutrinos into tau neutrinos during their 730 km journey from CERN to the Gran Sasso detector, observed a light-travel time of 2.4381723 ms while the expected time was 2.4382323 ms. This meant neutrinos traveling faster than the speed of light arrived 60 ns too early.

*Oscillation Project with Emulsion-tRacking Apparatus

Did we have to rewrite Einstein’s Theory of Relativity? I can reassure you, no.

In May 2012, OPERA repeated its measurement with a new, short-pulse neutrino beam. The final conclusion was that the CERN-to-Gran Sasso neutrino flight time matched the speed of light. The physicists concluded that a faulty fiber-optic timing element caused the original OPERA result.

The team compensated for the experimental blunder. In 2018, OPERA produced the long-desired result, observing 10 tau-neutrino candidates. The researchers confirmed that muon neutrinos produced at CERN can transform into tau neutrinos while traveling 730 km.

Back from “speedy” to ice-cold neutrinos.

To put it right away, the neutrinos aren't cold; Francis Halzen’s detector is. Instead of building an enormous artificial detector, he turned Antarctic ice into a gigantic neutrino telescope with thousands of photomultipliers watching for the tiny flashes of blue Cherenkov light produced when a neutrino interacts. Francis proposed the basic idea of an IceCube Neutrino Observatory in the late 1980s, and the detector now covers about one cubic kilometer of ice with more than 5,000 light sensors.

The important results are:

IceCube established that extremely high-energy astrophysical neutrino fluxes in the 100 TeV* to PeV range are coming from outside our Solar System.
*CERN’s LHC has a maximum collision energy of 13.6 TeV

Its findings opened a new form of neutrino astronomy. Light is absorbed or deflected, and magnetic fields bend charged cosmic rays. Neutrinos interact extremely weakly and travel directly from their birthplaces to Earth.

As a result, IceCube has identified individual cosmic sources. A particularly important source is NGC 1068 (Messier 77), an active galaxy about 47 million light-years away. Over the past decade, IceCube has detected about 80 (!) high-energy neutrino events.

With IceCube's results, scientists can now locate neutrino sources, i.e., cosmic-ray accelerators. Extremely high-energy neutrinos point back to these active galaxies containing supermassive black holes.

In short, Francis Halzen created a new astronomical instrument that began identifying the cosmic engines producing the most energetic particles known.

While neutrino oscillation experiments essentially asked: “What are neutrinos telling us about particle physics and stars?” Francis’s IceCube tells us something fundamentally new about the universe: “What can neutrinos tell us about the most violent objects in the universe?”

As Red Baron wrote before: Astrophysics is the new spearhead of physics.

**

Cooling at CERN

No, I am not referring to my cooling off following a full day of service at the CERN Open Days in September 2019.

This blog refers to beam cooling, or, more precisely, stochastic cooling of particle beams in storage rings. "Cooling" will narrow the transverse momentum distribution of a bunch of charged particles by detecting momentum fluctuations and applying an appropriate correction.

View of the AA ring. Slide from Simon van der Meer's Nobel Prize lecture:
Stochastic Cooling and the Accumulation of Antiprotons

In 2025, the Museumsgesellschaft chose CERN as the plat de résistance for its traditional summer tour. During our visit to the site, we saw a special cooling facility. The building we entered is familiar to Red Baron. Here, he helped to build the AA (Antiproton Accumulator) in the late 1970s.

Simon van der Meer invented and developed the technique of stochastic cooling at CERN in the Initial Cooling Experiment (ICE).

The principle of Stochastic Cooling.
Slide from Simon van der Meer's Nobel Prize lecture

Stochastic cooling uses the electrical signals produced by particles in a "bunch "to drive an electric kicker. This will kick the bunch of particles, reducing their transverse momentum.

The circulating particles (nearly) and the electrical signal travel at the speed of light. So the correction sent along the chord reaches the kicker well in time to apply the necessary kick to the bunch of particles.
When individual kicks are applied continuously over an extended period, the average tendency of the particles to exhibit wayward momenta is reduced.

 

Note the shrinking of the beam profile with time.

Cooling times range from a second to several minutes, depending on the required cooling depth.

Many accelerators had to work together.
Slide from Simon van der Meer's Nobel Prize lecture

The technique was used to collect and cool antiprotons* in the Antiproton Accumulator (AA). They were then injected into a modification of the Super Proton Synchrotron (SPS), i.e., the Proton-Antiproton Collider (PAC), to collide with counter-rotating protons.

*Noted as p in the graphic

While Carlo Rubbia proposed and pushed for the PAC, Simon van der Meer's stochastic cooling provided the necessary tool for its success. In 1984, Carlo and Simon were awarded the Nobel Prize in Physics for the discovery of the W and Z bosons that carry the weak nuclear force.

 
The ICE ring was also instrumental in evaluating electron cooling, a technique introduced by Gersh Budker.

Electron Beam Cooling is a technique used primarily in particle accelerators and storage rings to reduce the emittance (the measure of the spread of particle positions and momenta) of a beam of charged particles, typically protons or heavy ions. Dense, parallel beams of quasi-monoenergetic electrons travel with the heavier particles over the distance of the cooling section. The protons or ions undergo Coulomb scattering in the electron "gas" and exchange momentum with it, thus reducing the space coordinates and the angles of the heavier particles.

Click graphics to enlarge.

CERN operates electron coolers in its Antimatter Decelerator (AD) and in the smaller Extra Low Energy Antiproton (ELENA) ring.

Looking down on ELENA

This is where it gets exciting, because ELENA produces antiprotons of nearly zero energy that capture a positron, forming antihydrogen atoms. This opens research on antimatter.

Does a hydrogen atom fall in a gravitational field just as quickly as an antihydrogen atom? Are there differences in the emitted spectral lines of the two species? With the current measurement accuracies, no differences have been detected so far.

The CERN computer center was the other interesting part of our visit. Here, we were impressed by the volume of data from the experiments to be processed.

On the way out, Red Baron was abruptly confronted with his past.

On Display was the first paper by Tim Berners-Lee and Robert Cailliau, which marks the beginning of the World Wide Web, the precursor to the Internet. Even today, the sequence of letters HTTP (HyperText Transfer Protocol) for calling up websites is a reminder of the HyperText Project proposed by the two CERN computer freaks, who had no idea at the time that their proposal would change the world.

Click to enlarge

I had many a cup of coffee with Robert, a wide-awake colleague who was always up for a joke. CERN's superiors looked favorably on this coffee-drinking while leaning on bar tables, as new ideas were discussed with colleagues from outside their own field.

It was a thoroughly nostalgic visit to CERN.

**

Carlo 90

On March 31, Carlo Rubbia, Nobel Prize winner and former CERN director general, celebrated his 90th birthday. On this occasion, CERN organized a scientific symposium on October 18, which Red Baron, only 14 months younger than Carlo, attended.

On that day, I took the southbound ICE 271* from Freiburg to Zürich at 8:05, changed trains in Basel, and arrived in Geneva at 11:47.
*Intercity-Express

Some recollections of my encounters with Carlo flooded back during this train journey.

Professor Rubbia was my director general (DG) from 1989 to 1993, but I met him first in the late 1960s when he was already an ambitious physicist. At that time, CERN's high-energy workhorse was the 26 GeV Proton Synchrotron (PS), which spat its secondary particle beams into the East experimental hall.

Carlo was running an experiment in that hall and insisted on access to the area around his beamline. However, measurements of my radiation protection section showed that radiation levels were too high to allow access.

In those times, the radiation protection rules at CERN were as follows:

My section reported to the divisional radiation protection officer (RSO of the PS Division). The PS-RSO, Jack F., was a former British colonial officer with a stiff lip. His yellow fingers stuck out because he was a heavy smoker of unfiltered Navy Cut cigarettes. As a youngster at CERN, I liked and even admired Jack, for he spoke perfect Queen's English. We met over a cup of coffee several times during the week to discuss the radiation situation around the PS accelerator.

Suddenly, we three stood in the East Hall on top of concrete shielding blocks: Carlo requesting access to his beamline; the PS-RSO Jack F., denying it and, in his perfect English, arguing with Carlo; and me, the physicist who had delivered the measured dose levels. They exchanged strong arguments, but Carlo accepted my interjections because I was a physicist.

Eventually, the two men agreed to a compromise I could accept as the radiation-protection guardian. But the whole affair left a bitter aftertaste.

The second time I ran into Carlo was in a hallway in the spring of 1989. He stampeded out of his office and caught me, "Look what Fleischmann and Pons do in the States, and we all are sleeping here," waving the paper of Martin Fleischmann and Stanley Pons in his hands titled "Electrochemically Induced Nuclear Fusion of Deuterium."

In the following days, I provided him and other teams at CERN with neutron detectors so that my RP-Group could hardly fulfill its radiation protection tasks. And, yes, even Cold Fusion, if it works, will produce radiation!

To make the fusion story short, a CERN spokesman eventually said that "essentially all" attempts in Western Europe to reproduce Fleischmann and Pons's results had failed. This means that Cold Fusion did not, and I add that it will not work. Red Baron blogged on cold fusion in 2011.

My third encounter with Carlo was an indirect telephone contact in the early spring of 1990. I was on my way to a scientific meeting on radiation dosimetry at Gaussig near Dresden and had, exceptionally, taken my car. While on my way, following Germany's reunification, I wanted to see as much as possible of the now-accessible parts of my home country, regarded as Germany's heartland.

I was just climbing the Kyffhäuser monument when my mobile telephone rang. The DG's secretary called, "He wants to see you in his office at 2 o'clock."

I answered that I couldn't make it because I was traveling in Germany, but the DG could always contact my deputy at CERN. On my return, the latter told me he had discussed the query with the DG on the phone. Apparently, he had answered to Carlo's satisfaction, so I could finish my travel to Gaussig without further disturbance.

My last physical encounter with Carlo was during a presentation* I gave to the CERN Directorate about the presence of radioactive waste on our premises. CERN's host countries, France and Switzerland, requested a report on the quantity concerned.
*My first PowerPoint presentation

When high-energy particles strike matter, radioactivity is produced by nuclear spallation and neutron interactions. However, unlike radioactive waste produced by nuclear fission in power reactors, radioactivity from accelerator operation has low specific activity and a short half-life.

Nevertheless, the specific and total radioactivity in tons of steel, aluminum, copper, and concrete kept on the CERN premises and awaiting disposal is too high to be released into the environment.

The discharge of CERN's low-level radioactive waste into the national depositories in France and Switzerland is costly, and the Directorate must decide how to allocate the funds.


Back to my trip.

I took the streetcar to CERN and wanted to book a room at the CERN hostel, but my room was not ready. So, at around 1 pm, I left my luggage in a locker and went to the Main Building.

The auditorium was prepared but still empty. I chose my seat in the middle of row four to have a clear view of the projection screen during the lectures and to take photos for this blog.

I left my coat on the seat and went to the cafeteria to get something to bite on. When I returned half an hour later, the lecture hall was already well-filled. Surprisingly, I noticed they had reserved a seat for Carlo in row three just before me.

I took my seat and suddenly found myself surrounded by Italians. I have always admired, and still admire, not only Italian culture and cuisine but also Italian physicists, who at CERN have had the most brilliant ideas, following in the footsteps of Galileo Galilei's legacy. However, when it came to the realization, they needed the help of i tedeschi e i britannici.

Suddenly, the people in the row in front of me stood up. Carlo arrived and did lots of handshaking with his compatriots on his way to his seat, but he didn't catch my eye.

Carlo had headphones on, and I wore my hearing aid.

 I could barely see CERN's present Italian DG, Fabiola Gianotti. As chairwoman, she welcomed the symposium participants in English and opened the session.

Sir Christopher Llewellyn Smith, CERN's DG from 1994 to 1998, started the lecture series.

Carlo greets his successor as CERN's DG by raising his hand.

Chris showed a very selective curriculum vitae of Carlo.

He and Carlo first met in October 1982 as founding members of the SPSC*. Chris recollects, "I was immediately impressed by the enormous breadth of his knowledge of physics, detectors, and accelerators, his quickness - he grasped points before anyone else - and his originality and deployment of English."

*Super Proton Synchrotron Committee

In early 1977, Carlo Rubbia wrote a report on the feasibility of proton-antiproton (p-pbar) collisions in the SPS and, on March 1, invited 30 physicists at CERN and in its Member States to a study week to discuss a possible detector.

Opposition to the project came from a number of people who argued that at a time of economic stringency, CERN should not do something which would otherwise be done by Fermilab (USA) and others ...

(John) Adams countered this by pointing out that in the SPS machine, not only were the magnets more reliable, but the greatly superior vacuum would give a beam lifetime of 18 hours, compared to 150 seconds at Fermilab.

On June 8, 1978, CERN's Executive Board officially decided to proceed with the p-pbar project and UA1 (Carlo's experiment)

At 5 am on July 10, 1981, collisions were observed in UA1.

Meanwhile, on July 1, 1983, the HEPAP* pushed for a 40 TeV Superconducting Super Collider (SSC) in the US to "regain US leadership."

*High Energy Physics Advisory Panel in the US.

CERN countered with the LHC, which has only 14 TeV but a higher luminosity, i.e., the number of possible collisions.

This luminosity/energy trade-off had to be understood, but it was questioned whether a luminosity of 10³³ cm² s^-1 could be used.

 At the BNL* 1983 Summer School, Carlo said of a luminosity as high as 10³³, "It is a matter of learning how to handle it."

*Brookhaven National Laboratory on Long Island, NY.

Today, the LHC's luminosity is up to 2 x 10³⁴ cm² s^-1 and is expected to increase to 5 - 7.5 x 10³⁵ cm² s^-1 in the future.

After Carlo became DG on January 1, 1989, he stressed that compared to the SSC

- the higher luminosity would largely compensate for the lower energy

- LHC would also offer heavy ion collisions at unprecedented energy and ep collisions (the intention then being to leave LEP in place).

Nobel Prize winner Gerard 't Hooft during his video presentation

In 1974, the situation in physics was summarized as scenario zero: there is nothing more than the particles we now have in our models, which we call the Standard Model.

Gerard's Standard Model

What was needed most urgently was a direct observation of the W^± and Z⁰ vector particles.

Some theorists warned that the predicted particles would be too heavy to ever be observed, but Carlo emphasized, "Theorists should not worry. If these particles exist, we experimentalists will find them."

In 1982 and 1983. these particles were observed in p-pbar collision at CERN.

Gerard added, "I wish theoreticians could say, 'Experimentalists should not worry; we'll make a theory that explains what you are finding.'"

But here we are not very successful.

Astronomy - Red Baron would say Astrophysics is doing better.

Before the p-pbar collider became a reality, groundbreaking R&D work had to be done. Lyn Evans, former LHC project leader, centered his talk around the stochastic cooling of proton and antiproton beams. This "cooling" will narrow the transverse momentum distribution within a bunch of charged particles by detecting momentum fluctuations and applying a correction.

Here is a photo of the ICE* installation, the Initial Cooling Experiment at CERN.

* Another and different ICE

Beam cooling allows particle densities to be achieved sufficient to observe a high rate of proton-antiproton collisions. And cooling worked!

It follows a sequence of slides illustrating the discovery of the intermediate vector bosons.

The W bosons are named after the weak force. 

The Z boson is named after its electrical neutrality (zero charge).

Note the handwritten measurement protocol.

Carlo at the overhead projector

On January 20, 1983, Carlo announced the discovery of the W-Boson to the CERN community. The Main Auditorium was packed, and Red Baron was present.

Carlo's finger on the overhead foil points to the theoretically predicted value of the W-Boson.

Carlo said, "That is to be compared with the Weinberg Salam prediction, which is 82.1 ± 2.4 GeV/c², including all the higher corrections … So we would like to quote this number (m_W = 81 ± 5 GeV/c²) to be compared with that number (m_W = 82.1 ± 2.4 GeV/c²), which, by the way, is not so bad." Carlo followed his announcement with a relieved chuckle.

       

One day later.
In the middle, Herwig Schopper. He was my DG from 1981 to 1988.
On the photo on the left are Carlo and Simon van der Meer.
On the right, Erwin Gabathuler and Pierre Darriulat
of the competing UA2 experiment

Prof. Zoccoli garnished his presentation by quoting Rolling Stones' texts.

>

Carlo Rubbia and Simon van der Meer

CERN's former DG, Luciano Maiani (1999 to 2003), expressed what everybody felt.

Prof Zoccoli awards Prof Carlo Rubbia the INFN Medal
for the Anniversary of the Discovery of the W and Z Bosons.

French President François Mitterrand awards Carlo the Ordre national de la Légion d'honneur.

Given the many other awards Carlo has received during his academic life, the INFN Medal seems like a trifle.

Ultimately, Carlo's former collaborators came forward to frame the Jubilarian.

A close-up

Carlo's closing remarks ended in rapturous applause.

The crowd surged through the exit door on the way out to the reception in the auditorium's vestibule. Suddenly, he stood in front of me. I stammered, "DG, I was your guardian angel during your aegis. I was responsible for radiation protection."

He looked at me and countered, "I was responsible for radiation protection." He was so right. At CERN, the DG is responsible for everything that happens in the Organization. So, I hastily added, "I was head of the radiation protection group." He briefly smiled at me and then turned to a more familiar face speaking Italian.

I have not forgotten Carlo's ideas on an energy amplifier and superconducting power lines. These topics were discussed at the symposium, but a presentation here would overload the blog. I will perhaps report on this later. Here is a brief foretaste of the Rubbiatron, as presented in a blog in 2011.

**

The Enigma of Dark Matter

Like the previous lecturers on 70 years of CERN (here and here), Prof. Marc Schumann initially harped on the Standard Model but then turned to the cosmic microwave background.

 

After the Big Bang, the universe cooled down to 2.7 K. Yet an afterglow of the hot Big Bang can still be detected.

The measurements of this cosmic microwave background can only be reconciled with theoretical calculations by assuming the existence of Dark Matter.

Dark Matter not only exists but also shapes the universe. This property can also be simulated.

Therefore, we are looking for invisible and cold matter that does not or hardly collides and is stable. Dark Matter is "dark" because it doesn't emit or absorb light or interact otherwise. Why should DM interact with a detector?

Many theories of Dark Matter, including those that modify the general theory of relativity (the gray area on the right in the diagram), allow experiments and calculations to agree without assuming DM.

Since we cannot make any statement about the mass and type of DM, we must search for it in the entire energy spectrum. Experiments are currently conducted at the CERN LHC to detect dark matter in the energy range from keV to 7 TeV. In the lower energy range - Red Baron read zeV for the first time - Dark Matter should be ultralight.

One detector in which the University of Freiburg is involved is the XENONnT, located deep under the Italian Massif Central Grand Sasso. Measurements have been taking place there since 2020. Liquid xenon gas is an excellent scintillator and can be easily ionized.

XENONnT is an upgrade of the XENON1T detector. Prof. Schumann presented the audience with a world record for the new detector. It has the lowest background of all other DM detectors. The double beta decay of ¹³⁶Xe with a half-life of 2.1 × 10²¹ years and the neutrino double electron capture of ¹²⁴Xe with a half-life of 1.8 × 10²² years are clearly identified.

No significant Dark Matter signal has yet been found:
- in the XENONnT detector
- in the particle detectors at CERN
- in the IceCube detector at the South Pole

Outlook

The existence of Dark Matter proves that previously unknown physics must exist.
The detection of DM and the measurement of its properties are among the most important open questions in particle physics.
More sensitive (larger) experiments are already being planned.
- Broad scientific program also beyond Dark Matter
Additionally
- further Dark Matter candidates
- alternative approaches, new strategies

95 % of the universe is dark 

27 % of it is Dark Datter
-> does not absorb or emit light

We do not know what DM is

Various methods for searching: 

 -> scattering from atomic nuclei
-> production at the accelerator, e.g., CERN
-> Search for annihilation products

Challenge: minimizing background signals

So far, Dark Matter has not been found.

-->The search continues!

**