Heavy ion physics at CMS and ATLAS: hard probes

TL;DR

This paper reviews recent experimental results from CMS and ATLAS on hard probes like jets, quarkonium, and electroweak bosons in heavy ion collisions, advancing understanding of quark-gluon plasma properties.

Contribution

It provides a comprehensive overview of high-precision measurements of hard probes in heavy ion collisions at the LHC, highlighting recent experimental progress.

Findings

Enhanced jet quenching measurements in high-multiplicity environments

Detailed quarkonium suppression patterns observed

Electroweak boson yields consistent with initial collision models

Abstract

Hard probes are indispensable tools to study the hot and dense quark-gluon matter created in ultra-relativistic heavy ion collisions. These probes are created in the collision itself with a small cross section, and they serve as indicators of various properties of the medium, such as temperature, viscosity, energy density, transport coefficients. Hard probes measured by the CMS and ATLAS experiments at the LHC include highly energetic jets and charged particles, quarkonium states, and electroweak gauge bosons. An overview of those recent experimental results will be given that represent the path towards high-precision measurements, even in the challenging, high-multiplicity environment created by colliding heavy ions.

Full text

1 Transport properties, parton energy loss

In heavy ion physics it is common to compare A+A and p+p interactions in order to isolate physical phenomena unique to large colliding systems. For the interpretation of such comparisons, it is necessary to quantify the modification of parton distribution functions in nuclei, including gluon saturation at low $x$. Nuclei can be probed with p+Pb collisions at the LHC, and recent results in this area include high-energy photons and dijets with a large rapidity separation.

The ATLAS collaboration [1] has recently measured the nuclear modification factors, $R_{\rm pA}$, of isolated photons in p+Pb collisions, and concluded that the data disfavor a large amount of (initial state) energy loss, and impose constraints on the nuclear PDFs [3]. Dijets measured in p+Pb collisions with a large rapidity separation can probe partons at low x (between $10^{-4}-10^{-5}$). No broadening was observed in the azimuthal angle correlations for such dijets. Jet pairs, where both jets had a high rapidity in the proton-going direction (i.e. sampling low-x partons in the Pb nucleus) were found to be suppressed with respect to p+p collisions [4].

Departing from the baseline of nPDFs, one can study final state suppression using the nuclear modification factors in Pb+Pb and Xe+Xe collisions, as measured by the CMS collaboration [2] for charged hadrons [5]. On the left panel of Fig. 1 the charged-particle $R_{\rm AA}$ is shown for Xe+Xe collisions at $\sqrt{s_{\rm NN}}=5.44$ TeV for the 5% most central collisions, together with earlier data on $R_{\rm AA}$ in Pb+Pb collisions at 5.02 TeV. The data may indicate a slight difference in suppression at high $p_{T}$. Comparing $R_{\rm AA}$ values at the same number of participating nucleons, there is a hint of a greater suppression in Xe+Xe collisions, probably due to a geometrical effect.

The final state suppression of jets was also measured by the ATLAS experiment with an unprecedented precision recently, showing that the nuclear modification factor increases from low to high $p_{T}$ and from central to peripheral Pb+Pb collisions [6]. Since photons are not affected by final state interactions, the energy loss of jets can be more precisely characterized by selecting photon-jet events. The recent data published by the ATLAS collaboration [7] is corrected for accidental pairings and unfolded for energy resolution. The result can be seen on the right panel of Fig. 1, in terms of the distribution of the photon-jet $p_{T}$-balance distributions in different centrality bins, where $x_{J}\gamma=p_{T}^{\rm jet}/p_{T}^{\gamma}$. One can conclude that while many of the jets lose a significant amount of energy in the most central Pb+Pb collisions, there still remain some relatively symmetric photon-jet pairs, producing a peak-like structure close to unity in $x_{J}\gamma$.

2 Medium temperature, quarkonium states, heavy flavor

Quarkonium production is a sensitive gauge of the temperature in the colored medium created in heavy ion collisions. These heavy mesons have a modest binding energy and a large radius, and the Debye-screening in the quark-gluon matter may cause their dissociation. The weakly bound states (like $\Upsilon(2S)$ and $\Upsilon(3S)$) are expected to suffer a stronger suppression, in comparison to p+p collisions, than more tightly bound ones, like $\Upsilon(1S)$. The dissociation temperatures are predicted to be at $2T_{c}$, $1.2T_{c}$ and $T_{c}$ for these three mesons, where $T_{c}$ is the critical temperature.

Indeed, this successive suppression of the $\Upsilon$ states, measured in their dimuon decay channel was observed by the CMS collaboration using a high-statistics data set of Pb+Pb collisions [8]. The invariant dimuon mass spectrum can be seen on the left panel of Fig. 2. The result of the fit to the data including the three $\Upsilon$ states and the non-resonant background is shown as a solid blue line. The dashed red line represents the result of the same fit, but with the $\Upsilon$ yield for each state respectively divided by their measured $R_{\rm AA}$ value (i.e. their measured suppression with respect to p+p collisions recorded at the same center-of-mass energy). The suppression is also found to be gradually strengthening with increasing collision centrality. It was also shown by the recent analysis of the CMS collaboration that the excited prompt $\Psi(2S)$ is more suppressed in Pb+Pb collisions than the $J/\Psi$ ground state [9].

The $J/\Psi$ mesons also constitute an important tool to characterize the b-quark energy loss, since b-decays to $J/\Psi$ can be measured separately from prompt $J/\Psi$ production making use of the long lifetime of the b quark. The ATLAS collaboration has measured the nuclear modification factors of prompt and non-prompt $J/\Psi$ particles at high $p_{T}$, and found a strong suppression in both cases [10], as can be seen on the right panel of Fig. 2. Prompt $J/\Psi$ mesons are suppressed to a similar extent as inclusive charged particles, while the non-prompt states experience less suppression in the $p_{T}<20$ GeV range, owing to the more modest energy loss of b-quarks compared to light quarks. A further confirmation of this phenomena is that the suppression of non-prompt $\Psi(2S)$ and non-prompt $J/\Psi$ states, both originating from b-decays, were found to be equal. A similar conclusion can be drawn from a recent analysis of muons originating from heavy quark decays by the ATLAS experiment [11], and from the CMS measurement of the non-prompt $D^{0}\rightarrow K^{-}\pi^{+}$ mesons (coming from b-hadron decays), which exhibit significantly less suppression at low $p_{T}$ compared to charged hadrons [12].

It is also interesting to measure the $B_{s}^{0}$ state in Pb+Pb collisions to test if beauty and strange quarks can coalesce in the environment abundant in $s$ quarks, possibly leading to an increase of the $B_{s}^{0}/B^{+}$ ratio. The CMS collaboration has published the first result on that recently in the $B_{s}^{0}\rightarrow\mu^{+}\mu^{-}K^{+}K^{-}$ decay channel, with a possible indication of such an increase [13].

3 Jet substructure

After considering the spectacular jet quenching (parton energy loss) results obtained from heavy ion data, the next immediate question to ask is about the possible changes of the jet structure with respect to p+p collisions.

The CMS experiment has measured the transverse shape (energy density) of jets tagged by isolated photons, as a function of the distance $r$ from the jet axis for various centrality categories [14], as shown on the left panel of Fig. 3. A jet broadening can be observed for central Pb+Pb collisions in these photon-jet events, while no depletion is visible in the $0.1<r<0.2$ region, as opposed to inclusive jets. The longitudinal structure of jets, the fragmentation functions, were published by the ATLAS experiment [15], and show that there is an enhancement of particles with a small or very large fraction of the jet momentum, and a suppression of particles with an intermediate momentum fraction. The jet fragmentation functions were also measured in photon-tagged jets [16], and a ratio with respect to those in p+p collisions are shown on the right panel of Fig. 3. The jets with a photon partner, dominated by quark jets (blue data points), and inclusive jets (red points) are modified in a different way in central Pb+Pb events, although the interpretation of the data is complicated due to the different selection biases.

Finally, the CMS experiment has employed the jet grooming technique to remove large-angle, soft radiation, and extract the hard subjets. Since the opening angle between those subjets is sensitive to medium induced modifications, the distribution of the jet mass of these groomed jets is an important observable. These results indicate that available model calculations overestimate the yield of jets with a large groomed mass relative to the jet $p_{T}$[17].

In summary, the heavy ion research program at the LHC provides a precise and detailed set of experimental data, challenging many of the phenomenological models concerning jet substructure modifications, and supporting various expectations about nuclear PDFs, energy loss of light and heavy quarks and quarkonium dissociation.

Acknowledgments. The author is grateful for the support of the MTA Momentum program LP 2015-7/2015 and the support by the grants NKFIA K_18 128713, NKFIA K_17 124845 and NKFIA FK_17 123842.