7 or 8 TeV, a thousand terabyte question!

Event Pile-Up?

Event Pile-Up in the New Year?

A very happy new year to the readers of this blog. As we start 2012, hoping to finally find the elusive Higgs boson and other signatures of new physics, an important question needs to be answered first – are we going to have collisions at a center of mass energy of 7 or 8 TeV. While that may not feel like a such a drastic step up, certainly not like going to the full design collision energy of 14 TeV, it does bring its own sets of challenges for ATLAS. Understanding the detector performance is crucial for doing physics with our data, and we will have to make sure all the good work done during 7 TeV collisions can be extended if we run at 8 TeV. More collision energy means more pileup interactions; these occur when our detector can not distinguish between two separate collision events and thus considers them part of the same collision. We need to disentangle the pileup contribution to look at the real single collision event, and while a lot of work has been done in this direction, an increase in pileup is always a cause for concern. However, as someone working closely with Monte-Carlo tuning and production, I know firsthand how big of an issue this is going to be for us.

We need Monte-Carlo samples, or simulated data sets for every analysis, to calculate detector efficiency, backgrounds and what not. Also, a lot of times these Monte-Carlo event generators reflect our best understanding of certain processes, and we want to make sure they are predicting the behavior of real data closely. At times when they do not, we turn the knobs in the Monte-Carlo generators and tune them to match the data. Up until now, this tuning has been done mostly with 7 TeV collision data – although we tried to get the energy extrapolation right by using lower energy Tevatron and ATLAS data. We believe the simulation will do a good job at describing 8 TeV collision data – but we can’t be sure unless we actually compare, and most analysis groups will already want the latest and the best Monte-Carlo samples by the time the data starts coming in!

Then there is the question of size. The combined size of ATLAS 7 TeV Monte-Carlo samples is at least a few thousand terabytes. A very conservative estimate suggests we will need a few of months to produce the 8 TeV samples – the caveat is we can’t start producing them until the decision is actually made to switch to 8 TeV. This will happen immediately after the annual Chamonix meeting in the beginning of February, when the CERN management, engineers and experiment representatives meet to decide. As ramping up the energy results in higher cross-sections for the rare process we want to look at, from a physics perspective it is definitely beneficial, but we have to be ready to utilize this if and when it happens.

With input from Borut Kersevan.


Deepak Kar Deepak Kar is a postdoctoral research fellow with University of Glasgow. His physics interest is soft-quantum chromodynamics, and he is currently involved in underlying event analysis activities and Monte Carlo tuning in ATLAS.

Tweeting live #Higgs boson updates from #CERN

CERN Auditorium 2:15h before seminars began

My view of CERN's auditorium, 2:15h before seminars began.

“If it’s just a fluctuation of background, it will take a lot of data to kill.”

Dr. Fabiola Gianotti, spokesperson for the ATLAS collaboration, made this statement on Dec. 13, 2011 during a special seminar I attended at CERN. Within the minute that followed, I hurriedly concocted a tweet, tacked on #Higgs and #CERN hashtags, and sent Fabiola’s weighty comment out onto the WWW.

CERN, where the WWW was invented, is the main European particle physics laboratory. I was at the lab for a week to discuss physics and the  performance of the ATLAS detector, a 7000-ton apparatus used to examine remnants of high-energy proton collisions delivered by CERN’s 27-km Large Hadron Collider (LHC), straddling the Franco-Swiss border.

This turned out to be no ordinary week. The 2011 LHC program had yielded a fecund data sample, and we needed to take stock of our most promising new-phenomena searches. By far the most anticipated were those of the Higgs boson, hypothetical pieces in the emerging puzzle of the tiniest known subatomic particles. Signal rumours had been swirling around the planet in blogs and other media. I was asked by the media relations department at my home institute, McGill University, to live tweet the Higgs update event. I already knew our ATLAS measurements, but was keen to see results by our competitors, the CMS collaboration, running a complementary detector on the opposite side of the LHC. Exciting times!

We owe much of this excitement to Ernest Rutherford who, while a McGill professor of Experimental Physics early last century, unwittingly helped to kick off the Higgs hunt through his Nobel Prize work on radioactivity. Modern theories that posit the existence of one or more types of Higgs particles seek to unify – into a more symmetric and fundamental theory – two basic forces: 19th-century electromagnetism and Rutherford’s 20th-century radioactivity. As if that weren’t enough, observing Higgs particles would also help to reveal a mechanism by which various fundamental particles are endowed with their non-zero mass values. This gets at the very essence of the physical universe.

More recently, my McGill colleagues and I have taken part in the search for Higgs bosons using the Fermilab Tevatron matter-antimatter collider near Chicago. Just last summer at McGill University, Dr. Adrian Buzatu defended a PhD thesis using Tevatron data to set the world’s best limits on Higgs boson production in the low-mass region that is now revealing hints at the LHC. Adrian recently took up a postdoctoral position in our ATLAS collaboration, working with the University of Glasgow group.

In December, I entered CERN’s auditorium three hours before the seminar was to begin. Within about 30 minutes, all available seats and aisles were jammed. A mob formed outside the auditorium door, but security guards were able to maintain control. Unable to work in all the nervous energy and jostling, I tweeted, “You’d think it was John Lennon coming to CERN today.”

At long last the ATLAS and CMS talks began. Both collaborations had searched extensively for several different signatures, scenarios by which a Higgs particle could be created from LHC collision energy before disintegrating into lighter particles.

Given our detectors’ sensitivities, and the colossal Higgs-mimicking backgrounds, we knew in advance that our samples wouldn’t suffice for a statistically robust 2011 discovery. Nevertheless, both ATLAS and CMS showed suggestive traces in a variety of Higgs signatures. Enticingly, both collaborations ruled out overlapping mass ranges and recorded hints at similar masses.

These indications are thrilling. In this kind of science, discoveries take time and are often preceded by whiffs. We’re also cautious. Our 2011 results could be chance background fluctuations, tantamount to tossing six coins and getting tails every time. Only when our observations are flukier than tossing 20 tails in a row will we claim a discovery.

The 2012 data will likely enable us to observe or rule out the Higgs boson. Either of these outcomes would constitute exhilarating, 21st-century science. I look forward to tweeting about it.

For the event’s archived tweets, which contain links to further information, go to this page.

The above blog posting was adapted from an outreach article requested by the McGill Reporter newspaper. The ATLAS and CMS experiments have now submitted their Higgs search papers based on the 2011 data set.


Andreas Warburton Andreas Warburton tweets as @AWarb and is an associate professor in McGill’s Department of Physics. For information about his research and other interests, see http://twitter.com/#!/AWarb.