From Pauline Gagnon at Quantum Diaries: “If, and really only if…”


Pauline Gagnon
Pauline Gagnon

On December 15, at the End-of-the-Year seminar, the CMS and ATLAS experiments from CERN presented their first results using the brand new data accumulated in 2015 since the restart of the Large Hadron Collider (LHC) at 13 TeV, the highest operating energy so far.

CERN CMS Detector


CERN LHC Grand Tunnel
CERN LHC particles

Although the data sample is still only one tenth of what was available at lower energy (namely 4 fb-1 for ATLAS and 2.8-1 fb for CMS collected at 13 TeV compared to 25 fb-1 at 8 TeV for each experiment), it has put hypothetical massive particles within reach. If the LHC were a ladder and particles, boxes hidden on shelves, operating the LHC at higher energy is like having a longer ladder giving us access to higher shelves, a place never checked before. ATLAS and CMS just had their first glimpse at it.

Both experiments showed how well their detectors performed after several major improvements, including collecting data at twice the rate used in 2012. The two groups made several checks on how known particles behave at higher energy, finding no anomalies. But it is in searches for new, heavier particles that every one hopes to see something exciting. Both groups explored dozens of different possibilities, sifting through billions of events.

Each event is a snapshot of what happens when two protons collide in the LHC. The energy released by the collision materializes into some heavy and unstable particle that breaks apart mere instants later, giving rise to a mini firework. By catching, identifying and regrouping all particles that fly apart from the collision point, one can reconstruct the original particles that were produced.

Both CMS and ATLAS found small excesses when selecting events containing two photons. In several events, the two photons seem to come from the decay of a particle having a mass around 750 GeV, that is, 750 times heavier than a proton or 6 times the mass of a Higgs boson.

CERN ATLAS Higgs Event
Higgs event at ATLAS

Since the two experiments looked at dozens of different combinations, checking dozens of mass values for each combination, such small statistical fluctuations are always expected.

Top part: the combined mass given in units of GeV for all pairs of photons found in the 13 TeV data by ATLAS. The red curve shows what is expected from random sources (i.e. the background). The black dots correspond to data and the lines, the experimental errors. The small bump at 750 GeV is what is now intriguing. The bottom plot shows the difference between black dots (data) and red curve (background), clearly showing a small excess of 3.6σ or 3.6 times the experimental error. When one takes into account all possible fluctuations at all mass values, the significance is only 2.0σ

What’s intriguing here is that both groups found the same thing at exactly the same place, without having consulted each other and using selection techniques designed not to bias the data. Nevertheless, both experimental groups are extremely cautious, stating that a statistical fluctuation is always possible until more data is available to check this with increased accuracy.

CMS has slightly less data than ATLAS at 13 TeV and hence, sees a much smaller effect. In their 13 TeV data alone, the excess at 760 GeV is about 2.6σ, 3σ when combined with the 8 TeV data. But instead of just evaluating this probability alone, experimentalists prefer take into account the fluctuations in all mass bins considered. Then the significance is only 1.2σ, nothing to write home about. This “look-elsewhere effect” takes into account that one is bound to see a fluctuation somewhere when ones look in so many places.

Theorists show less restrain. For decades, they have known that the Standard Model, the current theoretical model of particle physics, is flawed and have been looking for a clue from experimental data to go further.

The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

Many of them have been hard at work all night and eight new papers appeared this morning, proposing different explanations on which new particle could be there, if something ever proves to be there. Some think it could be a particle related to Dark Matter, others think it could be another type of Higgs boson predicted by Supersymmetry or even signs of extra dimensions. Others offer that it could only come from a second and heavier particle. All suggest something beyond the Standard Model.

Two things are sure: the number of theoretical papers in the coming weeks will explode. But establishing the discovery of a new particle will require more data. With some luck, we could know more by next Summer after the LHC delivers more data. Until then, it remains pure speculation.

This being said, let’s not forget that the Higgs boson made its entry in a similar fashion. The first signs of its existence appeared in July 2011. With more data, they became clearer in December 2011 at a similar End-of-the-Year seminar. But it was only once enough data had been collected and analysed in July 2012 that its discovery made no doubt. Opening one’s gifts before Christmas is never a good idea.

See the full article here .

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