sPHENIX EMCal discussion

[Mickey Chiu <chiu AT bnl.gov>]

Hi Mike,

Here are my best stabs at some answers to your excellent questions.

On Aug 4, 2017, at 11:03 AM, Michael J. Tannenbaum <mjt AT bnl.gov> wrote:

Dear Mickey and sPhenix designers, 

     Since the CsI xTals from Babar are still available, I never understood why they were not proposed to be used in sPHENIX, which might have been  a big cost saving. This would also avoid the need for an Hcal in the magnet.

The CsI(Tl) has a slow decay component of 3.5 us, so there’d be this long tail into the following crossings after a hit, which would require some kind of baseline correction for the pileup effect in sPHENIX.  I think the collision rates for Au+Au might be as high as 100 kHz, or every 10 us, so there will be an effect.  Something similar has been done in Atlas, where there were tremendous baseline shifts in the LAr calorimeter that they managed to correct.  I’d put this in the not sure if the Babar crystals would work in the sPHENIX era but it certainly requires a bit of effort to see if it does.

Also I never understood, since the coil is 1X_o thick, I think, why the emcal in Babar  was not outside the solenoid as in CCOR, the first use of a thin coil superconducting solenoid in a collider. 

I think that’s a long 1 X_0 (20 cm), so there may have been concerns with electron identification from having some electrons produce spread out showers?  My alternative explanation is that they probably didn’t want to hear any more about CCOR, so they decided by fiat to do things in the complete opposite way of CCOR, without realizing that now they would hear even more about how they should have done it like CCOR ;-)

     Regarding the neutrinos. It is certainly a new way to measure a neutrino interaction, but the cross section must still be minute and the target tiny as shown, so it must take a huge number of neutrinos to make a measurement.

I believe the cross-section is larger than expected since the coherency increases the cross-section by Z^2.  But if you mean neutrinos have a tiny cross-section no matter what, then I agree, it would be difficult to measure at RHIC (not to mention the backgrounds that come along with collisions).

Mickey

Mike

On Aug 4, 2017, at 10:29 AM, Mickey Chiu <chiu AT bnl.gov> wrote:

Hi Edward,

You might recognize one of the authors, a certain Jason Newby :-)  He’s a former PHENIXian.  I remember attending the intermediate neutrino workshop which was here at BNL and thinking the coherent neutrino measurement was the most interesting proposal coming out of the workshop.  Then last year I happened to bump into Jason and talking to him about the prospects, which were scientifically very good at the time.  He mentioned how difficult it was to get ORNL management to understand the value of the science, something which I could very well understand.  So it’s good to see such great science successfully coming out despite all the difficulties.

I think one of the problems with your proposal is that you get a huge background from all the other stuff in the collision.  You’ll need to do this when there aren’t collisions, and with a strong source of low energy neutrinos.  At the very least.  I imagine you need also a very low noise readout, perhaps state of the art.

BTW, I mentioned to JH a while back that for ePHENIX we ought to install the Babar CsI(Tl) calorimeter as a nearly ideal barrel calorimeter for the EIC that fits within the e/sPHENIX base.  The CsI crystals as you know have a slow decay so they would take a large effort to correct for the inevitable baseline shifts into tailing crossings, but the EIC rates are so low that it should not be a concern.  The resolution (sigmaE/E) is  2.3% \oplus 1.4%.  There’s a proposed requirement by Alex Kiselev that you need 5% energy resolution at the EIC, so this would satisfy it.  I note that people in PHENIX have done studies that show that 12% resolution would be good enough, and I haven’t myself verified either of these numbers.  However, it’s clear that better energy resolution should always be better, up to the lower limit that radiative effects on the primary recoil electron impose.  Anyway, JH says that he talked to the SLAC responsible person for the Babar calorimeter and the crystals are available for ePHENIX if desired.

Mickey

-- 
Building 510C
Department of Physics
Brookhaven National Laboratory
Upton, NY 11973
Phone: 631-344-8428

On Aug 4, 2017, at 9:18 AM, Edward Kistenev <kistenev AT bnl.gov> wrote:

Hi Xiaochun, thanks for paying attention. I know Yuri Efremenko who found that hole under reactor but I always thought it is about neutrinoless double beta decays, this one is very interesting too even if neutrinos are effectively color neutral in coherent interactions. And it will be very difficult to prove that whatever flashes we’ll see in CsJ are not from neutrons which are plenty at RHIC too (except for the fact that all delayed - with respect to the bunch - flashes are from neutrons and any excess correlated with bunch crossing could be related to production of those same neutrons). 

Edward

On Aug 4, 2017, at 8:44 AM, Xiaochun He <xhe AT gsu.edu> wrote:

Dear Edward,

One of the authors of this project, from ORNL, is visiting GSU on the coming Monday and Tuesday. I could ask him more details if there is any details that you would like to know.

Best,

Xiaochun

--

Xiaochun He

Distinguished University Professor

 and Physics Graduate Director

Department of Physics & Astronomy

Georgia State University

Atlanta, GA 30303

---

From: sPHENIX-HCal-l <sphenix-hcal-l-bounces AT lists.bnl.gov> on behalf of Edward Kistenev <kistenev AT bnl.gov>
Sent: Friday, August 4, 2017 8:14:49 AM
To: sphenix-hcal-l AT lists.bnl.gov; sphenix-emcal-l AT lists.bnl.gov
Subject: [Sphenix-hcal-l] Ever-Elusive Neutrinos Spotted Bouncing Off Nuclei for the First Time - Scientific American

 

Here is what we can do with BaBar CsJ if we’ll find a way to install it and make it run in sPHENIX

https://www.scientificamerican.com/article/ever-elusive-neutrinos-spotted-bouncing-off-nuclei-for-the-first-time/

Ever-Elusive Neutrinos Spotted Bouncing Off Nuclei for the First Time

A new technology for detecting neutrinos represents a “monumental” advance for science

Juan Collar, a professor in physics at the University of Chicago, with a prototype of the world’s smallest neutrino detector used to observe for the first time an elusive interaction known as coherent elastic neutrino nucleus scattering. Credit: Jean Lachat / University of Chicago

Neutrinos are famously antisocial. Of all the characters in the particle physics cast, they are the most reluctant to interact with other particles. Among the hundred trillion neutrinos that pass through you every second, only about one per week actually grazes a particle in your body.

That rarity has made life miserable for physicists, who resort to building huge underground detector tanks for a chance at catching the odd neutrino. But in a study published today in Science, researchers working at Oak Ridge National Laboratory (ORNL) detected never-before-seen neutrino interactions using a detector the size of a fire extinguisher. Their feat paves the way for new supernova research, dark matter searches and even nuclear nonproliferation monitoring.

Under previous approaches, a neutrino reveals itself by stumbling across a proton or neutron amidst the vast emptiness surrounding atomic nuclei, producing a flash of light or a single-atom chemical change. But neutrinos deign to communicate with other particles only via the “weak” force—the fundamental force that causes radioactive materials to decay. Because the weak force operates only at subatomic distances, the odds of a tiny neutrino bouncing off of an individual neutron or proton are miniscule. Physicists must compensate by offering thousands of tons of atoms for passing neutrinos to strike.

The new experimental collaboration, known as COHERENT, instead looks for a phenomenon called CEvNS (pronounced “sevens”), or coherent elastic neutrino-nucleus scattering. CEvNS relies on the quantum mechanical equivalence between particles and waves, comparable to ocean waves. The high-energy neutrinos sought by most experiments are like short, choppy ocean waves. When such narrow waves pass under floating debris, they can pick out one leaf or twig at a time to toss around. Similarly, a high-energy neutrino typically picks out individual protons and neutrons with which to interact. But just as a long, slow wave would pick up the whole patch of debris at once, a low-energy neutrino sees the entire atomic nucleus as one “coherent” whole. This dramatically improves the odds of an interaction. As the number of neutrons in the nucleus is increased, the effective target size for the neutrino to hit grows in lockstep not just with that number, but with its square.

Of course, once a neutrino and a nucleus collide, the collision must still be detected. The neutrino bounces off and continues its inscrutable wandering but the nucleus also recoils slightly from the impact. That jolt kicks a few electrons out of their orbits around the nucleus and its neighbors. As the electrons fall back into place, they release their acquired energy as photons. Each burst of photons is the calling card of a neutrino.

Down Neutrino Alley

Although they are orders of magnitude more common than other neutrino collisions, CEvNS interactions pose formidable challenges to detection—so much so that no study has observed them since the mechanism was first theorized 43 years ago. COHERENT owes its success to its choices of neutrino source and target material—plus an unexpected assist from a cramped basement hallway.

The first problem facing COHERENT was the sheer tininess of a nuclear recoil. “Imagine that you take a ping-pong ball and you throw it at a bowling ball,” says Temple University physics professor Jim Napolitano, who was not involved in the study. “We know from conservation of momentum [that] a little bit of energy is imparted to the bowling ball. This [experiment] is detecting that bowling ball’s energy”—a signal on the order of 10 photons.

The challenge for COHERENT, then, was to find a material with atomic nuclei large enough for neutrinos to hit easily, but also small enough that they would noticeably recoil on impact. In addition, the material had to be transparent so the photons could reach the detectors. “That took me a lot of thinking—maybe 15 years,” says Juan Collar, a professor of physics at the University of Chicago and one of the study’s lead authors.

The second constraint was the neutrinos themselves. In theory, a recoil from a fast-moving neutrino would be larger, and therefore easier to spot—but if the neutrinos were too speedy, they would have too much energy to interact coherently. Eventually Collar and his colleagues realized that sodium-doped cesium iodine, a transparent crystalline material, would be an ideal target for the neutrinos that are produced as a by-product by the Spallation Neutron Source (SNS), a neutron-producing particle collider at ORNL.

<akimov1HR.jpg>SNS’s Beamline 13, which carries neutrons from the SNS collider to experimental stations. The same process that produces the neutrons also spits out neutrinos, which enter the COHERENT detector in the SNS basement. Credit: Jean LachatUniversity of Chicago

But using the SNS as a neutrino source added a third complication. Neutrons can be convincing mimics of neutrinos: They have no charge, so they do not show up on electromagnetic detectors, and they can strike a nucleus with the same effects as a neutrino. When the COHERENT team first tested the SNS grounds, says co-author David Reyna of Sandia National Laboratories, they found neutrons streaming out of the SNS’s neutron generation site, as expected—but also pouring through the shielding of neighboring experimental halls. The detector noise from neutrons was so bad that the researchers feared they might not be able to use the facility at all.

Fortunately, ORNL professor of particle physics Yuri Efremenko made a lucky discovery: a basement hallway beneath the SNS collider. Despite being close to the neutron source, it happened to be shielded by the densely compacted earth supporting the collider’s many tons of concrete. After negotiations with ORNL’s safety team, the COHERENT team removed the empty drums that had been stored in the hallway and set up shop in their new “neutrino alley.”

A Future Full of Neutrinos

Experts have only glowing words for the COHERENT result. Massachusetts Institute of Technology physics professor Janet Conrad says she is “really, really pleased.” Napolitano calls the paper “monumental” and “a huge accomplishment.” And no outside expert who spoke with Scientific American expressed any technical quibbles with the paper. With such a convincing demonstration of the CEvNS phenomenon, scientists can now turn from finding it to using it.

The biggest implication is having what Collar calls a “handheld neutrino detector.” The compact size will be a huge boon to neutrino researchers; Conrad notes that one of her previous experiments had “mini” in its name despite being 40 feet tall. (Larger detectors will still be useful for studying neutrino properties that cannot be measured with CEvNS.) Small detectors could also eventually assist the International Atomic Energy Agency in monitoring nuclear reactors for clandestine production of fuel for atomic weapons, Reyna says. The neutrinos pumped out by reactor cores cannot be shielded or hidden, so if CEvNS detectors can be adapted to spot these lower-energy neutrinos, inspectors could check remotely whether a reactor’s activities match its operators’ claims.

The CEvNS phenomenon itself also opens up new scientific frontiers. Collar and his colleagues are already working to test whether the rates of neutrino detection in different materials match theoretical predictions. Those same theories govern what happens in supernovae, which release 99 percent of their energy as neutrinos. That means further CEvNS experiments, in addition to detecting supernovae, could confirm or refute models of these colossal stellar explosions. And dark matter researchers are breathing a sigh of relief, because a close cousin of CEvNS could bolster ongoing searches for a hypothesized form of dark matter called weakly interacting massive particles (WIMPs). COHERENT’s discovery bolsters the viability of the WIMP theory, says Collar, and points the way to future detection technologies.

All these potential advances give physicists plenty to be excited about, says University of Michigan physicist Josh Spitz. “This [study] is just the tip of the iceberg. There’s a whole lot more interesting stuff to come.”

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